"Not unless I get a better offer!"

It is actually true. You're not leaving unless you get a better offer and until you hand in your notice. That is as true today as every other day.

You can say as much, and your boss will probably ask "Are you looking?" - but it's normal and acceptable for employees to keep an eye on the job market, and for recruiters to contact you (via LinkedIn or from an old CV, or any other method). You should do this anyway, to ensure you're being paid something close to the market average, and to track which skills are in demand.

He may ask whether you've been to an interview, which you can probably answer truthfully. Eventually he'll be asking "Would you leave if someone offered you more money and/or better conditions?" to which you could truthfully answer "well, duh!" ;-)

This question falls in the category “If you don’t want to hear lies, then don’t ask this question”. Your answer is “No”, or “No, what makes you think that”. You should give exactly the same answer as someone who doesn’t have the slightest desire to leave.

Why is it Ok to lie? Because the question shouldn’t have been asked. And because you need to lie to avoid having unjustified problems. It is entirely your right to tell your boss that you think about leaving the moment you give notice, and not one second earlier.

I've been in this situation before, and honestly I regretted caving in and telling them. If I had to do it again, I would rather flat-out lie than tell the truth; as unprofessional as it may be to lie, it's also unprofessional for them to put you in that position. Regardless, here are two ways you can get around this without barefaced lies.

With jokes

In an informal conversation, if you want to avoid outright lying, you can use humour to imply things without actually saying them.

Something like "Actually I feel quite tied to this place. If only because no one will pass me the scissors to cut the rope" might work.

Or you might be a little daring and liken it to a relationship with a jealous lover: " I thought you trusted me, . If you cant trust me, maybe we should see other people". The added metaphorical ballsiness of this joke might work even further in your favour, but definitely make sure that you know this will be taken as a joke before trying!

It catches your boss off-guard, makes the conversation feel a little less like an interrogation, and it implies that you want to stay. Remember, it's not a lie if you're telling a joke. Humour can let you get away with all sorts.

With deflection

If it's more formal, to the point where you can't appropriately work in the humour aspect but still don't want to bald-face lie, talk about the things you like about this place but don't word it in a way that makes it obvious that you are deflecting.

An example might be: "You know, I'm actually okay, I mean the people are nice and the perks are useful". All of those things might be true, but none of them mean that you are or aren't looking elsewhere. "I'm okay" is not "I'm staying".

I appreciate this will all feel like mental gymnastics at this point, but it's your best shield against this type of boss.

I've been in a very similar situation in the past with a manager who seemed to have no boundaries. I'm also not a great liar. I was asked if I was intending to leave, to which I responded:

I'm not actively looking for a new job.

The word actively was key to me being able to say this truthfully.

When pressed further by my manager, asking whether I'd take another job if it were offered, I said:

I'm sure you'd agree that I'd be foolish to sabotage my career by passing up a great opportunity. I don't have anything on the table at the moment, but you would be the first to know if I decide to pursue a job elsewhere.

If you're presented with more questions around this, from different angles, repeat the same thing with different words, using phrases such as:

I'm not interviewing at the moment

I don't plan to leave

I don't have any plans right now

I would let you know immediately if anything changes

It can also be really useful when somebody is trying to pry information from you to prefix your response with:

Like I said...

This indicates that you're not going to be browbeaten into saying something you don't want to.

I suppose this does fall under the umbrella of being evasive, but it avoids lying outright, which can be easier to pull off. I'm aware that it's similar to the current top rated answer, but thought some other phrases could be useful.

he's trying to catch as much warning of unhappiness graduating to active job search as he can.

Isn't this a good thing? You don't mention what you think he will do if he figures it out. You also didn't mention if you've given him the chance to understand why you're unhappy and are working on leaving.

Unless you're expecting to be fired on the spot (I presume you're American) what would the harm be in simply engaging him on his suspicion and saying: "Good of you to notice my unhappiness, here's what I'm unhappy about"? By focusing on that instead of your actual plans to leave, you can give him insight into what he could improve.

Your conclusion that people seem to advise to say nothing may be missing the point. There is no (or shouldn't be) any harm in giving your boss a chance before moving on. Once you've given them a chance and they don't listen, sure. But if you've mentioned your discontent before then I don't see why your boss would be 'cornering' you to get you to tell him why you're unhappy.

You can be as forward as him without lying:

If he ask you if you are unhappy and why you can get into detail about this, this is regular stuff so far.
When it comes to:

Are you considering leaving the company?

Just answer:

Sorry but I'm not willing to talk about that, I've already mention what are the things that bother me and can be improved, we should focus on that. If eventually decide to leave I'll let you know in the notice period agreed in our work contract.

The fundamental problem here is that anything other than a complete and positive refutal, will be taken as "yes I am thinking of leaving".

Lying is wrong, and invariably comes back to haunt at a later stage - for example in this situation could lead to a rapid deterioration of trust during your notice period.

I think the best approach if faced with this situation, is to use a combination of humour and deflection. For example:

What kind of a question is that?!

...if said with the right tone, should lighten the atmosphere, hopefully remain ambiguous and so defuse the situation.

Ultimately though, I'd say it's better to avoid getting into this situation unintentionally, through wise behaviour and discretion.

Simple, you just need to look your boss in the face and say "No, why would I do that". Maintain eye contact the entire time.

This essentially puts your boss in a position where your boss has to either admit to doing things poorly or convince you as to why you should leave the company. If your boss is smart they'll leave it alone. If your boss isn't then you can just follow up pretty much any response to that question with, "Sounds like you really want me to leave".

Any boss who, "gets things out of people" should be handled in a polite and cut throat manner. They've earned it.

There is no good solution.

There is no answer that allows you to be honest and also keep your hand hidden. Either you're telling the truth or not. There's no middle ground here.

There's no version of this that plays out that makes you look good. If you lie, you destroy some of your professional reputation, but you get to leave on your terms.

If you tell the truth then you might be fired in advance or phased out in response, but you keep your professional reputation in one piece.

You COULD maybe find a middle ground. Having a meeting and making clear that you intend to leave in the foreseeable future and help the company find and train someone before your deadline.

I don't know the best answer, but it's clear from the break down of the question is that there is no GOOD solution. There's just varying degrees of discomfort.

My answer to "are you quitting"

No, but I'm not happy here and I can't see myself staying here much longer.

.. and then follow up with a list of things that are making you unhappy.
This is assuming you can be convinced to stay.

If you cannot be convinced to stay then:

Yes, but I'm not going to abandon you. I'd like to work with you on finding my replacement and even some training if they need it.

This is a fairly diplomatic and responsible answer.

My issue with lying to an employer, is they're paying to do work. But if you're there on a pretense of essentially running out the clock till it's time to go, that's... disingenuous.

Final note: We need to stop operating in bad faith. There are many managers out there who get it. Who get that we need to grow as professionals, not everyone is out to get you and maybe assuming good faith isn't such a bad approach. Maybe this manager is just trying to keep the ship on course and if that's the case, he can also be a really great ally in your transition or in getting new opportunities with a referral.

While other answers are helpful, I'd add that telling him outright that the line of questioning is not appropriate should stem the badgering/pursuit he is expected to do.

You may have to answer briefly first with a No or something evasive, and then follow with halting further questions. You still need to choose your own words in doing so, but choosing in advance may make you more comfortable going into that meeting. Highlighting that he should stop seems like something he won't be expecting or prepared for.