There almost has to be some kind of silly, convoluted answer for this, right? Why would someone be in her apartment, already waiting for her, and then stay in her room the remainder of the day, with the MC never seeing or hearing a peep out of them when we've established that it's very easy to hear through the walls?
I mean, it could be played straight and what he thinks is happening actually is happening, but the situation is so dumb right now that I'd be a little disappointed if that was the case (note that I don't mind at all if this is actually NTR, just that this setup for it is silly).