I guess we're not getting any more reasoning on why everyone thought the bath was empty, because we're on to the other obvious plot hole: Why Kuma chose to believe seeing her sister dripping blood and giving off a psychotic smile meant she was clearly having fun-fun sexy time.
...with this page and the previous one I can conclude one thing - Kuma is into really kinky shit.
Well, her idea of "notice me sempai!" on behalf of Kiso was to try to strip her naked (by landmine if necessary) on a few occasions, and we all saw how Hibiki and Inazuma wound up in this artist's eyes, so, yeah, I'm not exactly surprised.
--THE ADMIRAL?!That aside, how did you wind up like that -kuma?Even if it was your first time, you wouldn't...Oh, I see, so that was just--It seems like an old wound of the admiral's reopened.On his head...What games were you playing -kuma?!