I have to disagree with COil on this one, after talking to Thom about this, one day.
Stalkers may re-enter stalking if all conditions for doing so are met. If you have a squad of stalkers, their entire squad is revealed at once, and must re-enter at once.
Now, I am not saying that I am right. I am saying that this is how I understand the ruling. For purposes of this discussion, someone, please, remind me of a unit that has Stalk and that can have more than one model in that squad.