The scene could happen literally anywhere, any time they don't realize he's there. That could be anywhere. There's absolutely no reason to set the scene specifically in a bathhouse other than a stupid "lucky pervert" cliche for pathetic gooners.You mean she should say that when he was right next to her?
Since we want to play this game, she told that in private when nobody else was around, she did that because in the next chapter she says at the same time, its to giver her a push but if she doesnt act, someone will take him ... like her, that means she is willing to give her "rival" a push but still hopes he picks her.
What other locations? either he wouldnt be present or she wouldnt engage in this conversation, its a "overhearing" cliche I seen many times before, you rather it was in a hot springs? but then we would get into the argument about if she would say anything if there was a chance of him overhearing and there wouldnt be at the very least for the part she talked to herself.
Also we seen again and again, hot baths and springs are common used for socializing so there is nothing strange about this, its all about some people not liking the place for reasons, you rather nothing happened at all? I will say there is a rather interesting bomb dropped at the end of next chapter that kinda explains why this taken place before it.
Here's just one example that you could have probably thought of yourself if you bothered to try harder than the author: empty classroom after school. Same scene, no tits necessary. Easy.
I'm starting to feel like you're being deliberately obtuse, so I'm not going to bother trying to explain this extremely simple thing to you any further.