Got into a situation that the PC's were all held by a high level Hold Person spell outside of combat. I allowed them to roll to escape every few seconds, and finally one of them broke free during the villain's monologue.

That one player wanted their PC to appear as though they are still held by the spell, using stealth or performance to do so.

I was under the impression that spells like this let the caster to know if/when someone breaks free from it. During combat, it'd make sense for the caster to know that their target is no longer under the effects of things like Blind/Deafen, and as a DM you inform the players when their opponents have broken from a condition caused by a spell.

Is that the case only because everyone is hyper-aware during combat, and can see that, for example, a character is no longer blinded from a spell's effect? Or does the caster intrinsically know the moment their target breaks free?

tl;dr do casters know when their target breaks free from a spell that causes a condition?

If the player is trying to deceive the caster, then yeah, there is an appropriate skill check for that.

The rules don't specify so it would be a DM call. Decide it whichever way you like and go with it. However, whichever you choose you should probably try to remain consistent.

If the NPCs can't know when and if a spell ends then the players won't know either.

If the players cast hold person on the opponents but then one of the other opponents casts dispel magic, the players won't know that the effect has ended on that opponent.

It works naratively in the current situation and is pretty cool but you might want to think about how you want it to work in the long run and perhaps ask your players if they have a strong preference on how the rule should work for the shared game world.

I might also distinguish between spells that require concentration (e.g. hold person) and those that don't (e.g. blindness). If you are concentrating on a spell and the target escapes from the spell, does the spell continue without effect? I would tend to think that the caster would become aware that concentration was no longer required since there were no or fewer targets left. On the other hand, a spell like blindness doesn't require concentration, so you could imagine that a caster might be unaware that a creature has recovered until they give it away.

However, that would be a ruling since RAW doesn't seem to say.

P.S. You could also go with the caster being aware of all spells ending - their spells affect the Weave of magic in the world, the caster is aware of the weave and the changes they make when they affect the weave. Casting any spell changes the weave and it is possible that any caster remains aware of the changes they make so if they end they are aware of it. They would also be aware of their concentration either changing or no longer being required. However, this is also a ruling (if a different one) since RAW doesn't specify what happens which leaves it up to the DM.

This is one of those slippery slope kind of things. In the situation you described, I'd want to let the character do that because it's cool.

But a certain kind of group will take that idea and run with it, potentially causing trouble for you later. Hold Person requires concentration, so breaking the spell and 'pretending' it's not broken could prevent enemy spellcasters from using any other concentration spells, or wasting their actions on spells that no longer exist. Suddenly a high Performance or Deception skill approaches the power of an anti-magic field, and it also can make your millennia-old archmage lich look like an idiot that failed out of Hogwarts cause he doesn't even know what's going on with his own spells.

Personally I see concentration as linking a caster to the spell effect. If that spell breaks, so does their concentration. And they should probably notice that. But when your spell is affecting 4-5 people and one breaks out, it's much less clear what that might feel like, and more believable that it could be overlooked.

All that being said, I don't think it's unbelievable that a character's attention to such things could lapse during a passionate monologue. I'd make it clear that it only worked in this case because the villain was getting carried away with their own vision.

I would go with Rule of Cool for this.     Depends on the spell and I would be strict with skills.  That is the player does NOT get to choose stealth or Performance.

Personally, I would do the following:

Hold person = Paralzyed.

That means a Performance check vs the Passive Perception when they pass their save and also at the end of their turn.   While they are performing, they still get all the penalties of Paralyzed.   You can't move or speak while pretending to be Paralzyed, so you fail any Strength or Dex saves, etc.

Then,  if they wanted to do something WITHOUT being notice, on their turn I would have them make a Stealth check ON TOP of the Performance check they make at the end of their round.   DC again vs the Passive Perception check to take any action without being noticed.  Note most actions that they take while being stealthy automatically end the stealth.  That is,  if you are trying to stealthy move, cast a spell with a verbal/somatic component, etc. then after you succeed you are no longer being stealthy and that means the bad guys knows you have made your save.