It would make sense...there are no exits near me--and there's also no mist over here, yet. [There's a rustling as he has to get another roll of receipt paper out of the drawers to write up his notes.] Maybe ask Kacchan next time he comes around to tell you how close you are to any doors.

[Izuku is humming, considering the thud. Maybe someone had left a door open accidentally which was letting the mist slip in. Or maybe...

...it could be whatever it was in the mist again. If real people went outside at any point, then maybe...

...ugh. He didn't want to think about it, and it wasn't productive to his brainstorming right now.]

Maybe whatever caused the sound let some mist creep in. [Which was somehow the best and most tame option he could say.]