[MENTION=6857451]ThePolarBear[/MENTION]
Thanks a lot for the grammar course. Blame auto corrector in an other language.

On my cell phone these grammar errors can and will happen.
None of my examples are flawed (at least from my point of view)
People here apply the sneak action to someone who is not sneaking, naming the warlock!
He is not trying to sneak past anyone. He is just standing there, not moving, trying to breathe as silently as possible.
Does that count as a stealth check? Nope. So no action has been taken.
Can he be detected with a perception check? Of course he can.
Rules on invisibility are crappy at best so are the rules on perception vs sneak.
If you use common sense and RAI, you will rule it the same way I did.
1) PHB 192, Hide section. "When you take the Hide action..." You forgot to add this in your analysis, and then...
None of the warlocks in my examples are taking the hide action. None of them are moving. None of them are trying to be sneaky. They just stand where they were, hoping that they won't be attacked. The enemy only makes a perception check to confirm that the target is still in place.
Facts:
1) The enemy knows where the warlock was prior to his disappearance.
2) The enemy does not know what power enabled the warlock to disappear.
3) The enemy is thus not aware that the warlock can't move.
4) The warlock holds his breath. Thus trying to negate any sounds coming from him.
5) The enemy can always strike the zone where the warlock was last seen.
Now this part must be clarified again. The warlock has not moved. But there is no way to be absolutely certain that he is still in place. There are too many ways to be invisible that let you move after the fact.
6) Yes the enemy will try a perception check to see clues of the warlock's whereabouts. A perception check means that the enemy is looking for footprints, breathing sounds or some other clues. None of these apply to our warlock since the warlock is not taking the hide action but he is trying to hold is breath or he is trying to breathe slowly. Thus we have to set an arbitrary DC.
That DC will be high or low depending on what's going on around.
On the other hand.
If you considered that you can immediately find and invisible, unmoving and suspended object with no perception or investigation checks; then invisibility means nothing at all except to give disadvantage on attack rolls. For some people, if you follow the RAW, that is exactly what it is. Not in my book.
DM: You get in the room. You immediately notice an invisible sword on the ground. After all the sword can't take a hide action...
Sometimes, the RAW is right. Sometimes the RAI is way better. It's a DM call to judge.