@Simpleton
I agree that it is rather unlikely that he wanted to spy on them, it is just interesting that he did it that way instead of say listening a bit in from the distance to hear if he recognizes for example the voice of the teacher and then make some owl noises or something to give her a...
What came to my mind a while after reading this chapter: Did the guy just transform into a tanuki after hearing girls playing around in the onsen so he could spy on them? He should have been able to hear them roughing around even if he wasn't a ninja after all.