The spare key you hold is a responsibility to respect, not permission to arrive unannounced.

Holding a key is a piece of trust rather than an entitlement. It was given for emergencies, deliveries and the days they are away, and the owner still expects to know who is in their home and when. Arriving unannounced because you can turns a safety measure into a loss of privacy. What you were handed was access, not permission.

Message before you use it, every time, even when you are certain it is fine. If you need something inside, ask rather than letting yourself in and explaining afterwards.