If you don't do anything wrong, then you're ok.

Technically, the cameras could be anywhere, if there are any cameras at all, but I've never heard of a camera being in a mirror. Though I guess you could technically make anything into a camera these days. No idea. But if avoiding mirrors make you feel slightly calmer, then continue to avoid them. No sense in subjecting yourself to torture.

Try to take deep breaths. I can't tell you if you're being watched on camera or not, but if you are, remember that you know and trust these people. They won't hurt you; they never have and they never will. You are safe in their house, as you have been each time you've dog sitted.