[Firebird]

I have only come over something like that once and that was in a spa /capsule hotel in Sapporo where they had cameras in the changing and sleeping area. This was mainly to avoid theft i assume. As there was no ladies area in this Spa i cant say anything about that question.
I have been to many onsen and it is defenetly not common, so dont worry.
But you´ll find cameras nearly anywhere in Japan if you start searching them...