This is actually a great mental exercise for determining if you're fundamentally unhappy at your current job. Just try and imagine your boss calling into a meeting like the one the OP describes and giving you some sort of, "your work has been good, but unfortunately we have to make some changes, and this will be your last day" speech, and imagine how you would feel.
If your reaction is something along the lines of relief, then you are fundamentally unhappy at your job. You have not only concluded that it is not a good situation, but that the situation is incapable of improving. It's likely only artificial mental restraints keep you from doing anything about it ("oh man I may have to move, moving is annoying... and I guess I get paid pretty well, most of my friends are making half what I do... and my boss said some things would change, although he said that months ago...") and those restraints aren't even that strong, otherwise you wouldn't be relieved if you got laid off.
I'm not sure if that mental exercise is something I would trust for myself. I'm extremely satisfied with my job, but when I run through the exercise of imagining myself getting fired, there's a reflexive twinge of excitement about what I would do if I were immediately thrown into such a scenario.
I don't think that I could conclude form this that I'm "secretly unhappy" with my job. I think it's my innate longing for wayfaring adventure, even unwise and irresponsible adventure, that I constantly have to reign in to keep myself productive. It's excitement I imagine feeling, not relief, and in reality I'm sure anxiety would accompany (even drown out) the excitement.
The most important thing, of course, is knowing yourself.
I mean, I have that same twinge about a lot of things, and I had identified it as something I need to evaluate instead of acting on immediately. This lead to me sticking in a position way too long (I actually only recently resigned), and becoming utterly miserable.
The line is fuzzy, but there was definitely a point at my last position where I would have been premature in leaving. I would say, however, that the appropriate point to leave would have been right around the time when relief would have been my primary reaction to being let go.
If you haven't been in that situation, it's a bit hard to relate the feeling, but it's definitely it's own class of "positive", not like the "excitement of reacting to the immediate and having to think on ones feet" feeling at all.
I personally think that the biggest problem with this approach is that you have to remember to do it. The "fundamentally unhappy" situations tend to sneak up on you as you get into that weird cycle of going to work and doing the dance being prioritized higher up on the list than your long-term mental health. It's hard to stay in the moment.
It's a useful exercise from the other direction, too. As an employer or manager, ask yourself how you'd feel if Employee X walked in your office and handed you their resignation. If your gut reaction is closer to relief than to horror, then terminate Employee X sooner rather than later.
Those restraints are stronger than you think, in all honesty. Especially if you have any insecurity whatsoever about being able to find something else or make it on your own.
Once you can get beyond that insecurity, though, your feelings change entirely.
If you need to do exercises to figure out your unhappy then that should be sign enough. It means that you are battling layers of denial - not a good thing.
If your reaction is something along the lines of relief, then you are fundamentally unhappy at your job. You have not only concluded that it is not a good situation, but that the situation is incapable of improving. It's likely only artificial mental restraints keep you from doing anything about it ("oh man I may have to move, moving is annoying... and I guess I get paid pretty well, most of my friends are making half what I do... and my boss said some things would change, although he said that months ago...") and those restraints aren't even that strong, otherwise you wouldn't be relieved if you got laid off.