Scott knows someone's in the club, and that someone's being awfully careful about how they walk and move. That makes it unlikely to be someone who stumbled in -- okay, that was unlikely, anyway, but it makes it even less likely. He hangs back in the shadows until he can identify the man who's moving toward the front of the club. Then, and only then, does he put away his weapon and step out.
"You should be careful about who you sneak up on."