[Hmmm... That's a fair argument, and Rin considers it.]

It was just the first stairwell, wasn't it? Technically, they wouldn't have had to go up the stairs, or even inside it at all. They could've just opened the door, thrown the stun grenade in, and took off while everyone came over. Then all they would have to do is act like they were drawn in by the noise like everyone else.