Thanks to all for your comments! They really help me out. I were forgetting about some rules (like the -5 if you move more than 2 squares).
My goal here really isn't to denied this tactic, you're right about the player investing ressources to be able to do this, but I want to know the limits of this tactic.
In all cases, stealth or not, we can assume than a warlock that move at least 3 squares gets bassically +2 to defenses (well, ennemies get -2 to attacks, but that's the same). It's already a great feature in itself.
Then, for the stealth here's some clarification that I read inspired by some of your posts.
Even if you're in light, by RAW, you only can't hide if you are carrying a light source (p.188) (but I really have the feeling I've read somewhere else that if you're standing in a light source, you can't hide... can't find where though).
You gain the concealment as soon as you moved 3 squares (the rule state : on your turn, if you moved at least 3 squares, you gain concealment; would have been stated : at the end of your turn if if was intended otherwise), so you could effectively : attack (standard action), move at least 3 squares, hide in plain sight since you have concealment (at -5 because you moved more than 2 squares). Forget about ennemies knowing which square you're in. The warlock hides after 3 squares (hiding is "as part of the move action" so it don't use an action on itself), but he still can move up to his move, so there's as much chance that he's in the 3rd square of his move than in the 4th, 5th or 6th (well, in my case, the warlock in the group is a halfling, so 4 square max... but still).
Ennemies aren't making perception check. They use their passive perception score. They have to use a standard (not a minor, as been said) action to make an active perception check (p.186).
So this is really becoming a problem.
Warlock attacks, move at least 3 squares, make stealth check.
Monsters turn : those who want to attack the warlock have to succed an active perception check if passive wasn't enought... but it takes a standard action, so if your passive perception score isn't enought, you just CAN'T attack the warlock.