My question is... is he hiding the tent FROM HIS OWN PARTY? If the assassin was a guide, trusted to keep watch, wouldn't it be normal for him to know where everyone was sleeping? You know, in case he had to wake them up...
It sounds like with everyone eager to stand watch, your party doesn't sleep much. I get the sense that the players knew or suspected something was up, and metagamed to protect their characters.
I think it's reasonable, for the rest of your campaign, to rule that whoever is on watch has to make Perception checks to find his teammates if the party is ever attacked at night. That is, in the unlikely event that said teammates actually sleep.