Hidden surveillance of the unsuspecting in restrooms, bathrooms, and bedrooms -anywhere where people can expect to have privacy- should of course be illegal. Nannycams in living rooms and other "public" rooms in private homes probably should be legal for obvious reasons.
Yes, secret cameras can get the host in trouble, but poke around at airbnb listings and you'll see cases where everyone reports highly visible internal cameras.
If an vacation rental landlord planted secret cameras and...
Bad news all around.