|It's only an invasion of privacy if the monitoring is done in secret.

Uhhh... that seems very incorrect. If someone pokes their head into your shower session, it's an invasion of privacy - whether or not they let you know they're peepin on ya.

The equivalent here is if it's a company shower, and your supposed to be cleaning an office appliance, not yourself. In that context someone poking their head on to see how it's going is fine.