I dont trust the phrase from the guy, I do trust the message ANY stale draft will tell you when you click it. Look for that message, though it sounds like you have enough. If not you do have to clear the room anyway to pop it.
Semi off topic, but not deserving of its own thread:
I never tried but always assumed that whoever poped lost all of their hate the second they traded. If thats true, the formor in the room should no longer aggro even w/o sneak up (eg formor do not aggro to 0 hate) and you can pop and pull without needing to clear the room first.
Not supprisingly I've never managed to convince anyone to risk a codex to find out, has anyone tried this and can confirm/deny that it works ?