I was the hotel's lawyer and she was showing me a suite where someone had gotten hurt so that I could see the layout of the carpet/tile etc. She got the key to the empty room from the front desk and she knocked before we went in.

That was 20 years ago, and every time I'm sitting in a hotel room and I hear the sudden "ka-chunk" of a staff member trying to enter the room to replenish the minibar, fix the toilet, collect my luggage, etc. I wonder how hard can it be to train all staff on their first day of work "Always knock before entering a room. Always."

A 100% success rate on knocking avoids accidental shootings and nude guest sightings and complaint phone calls of "your staff entered my room without knocking give me a free night."

For what it's worth, I find that American hotels are better at knocking and respecting Do Not Disturb signs than foreign hotels, where locked doors and DND signs are sometimes treated as an invitation to enter.