Two things....

The Marriott College Park, MD, several times I had late checkout, *AND* a DND sign on the door, and housekeeping tried to enter my room. And other people I know had this happen as well. One woman did not have her door bolted and was dressing as they walked in. Quick knock then they just come in. Don't even give you the opportunity to call out or come to the door.

The Marriott Marquis in DC, one time I had my DND sign on the door (I had forgotten it was on there), and I called to ask that my laundry be sent up. After a while, I received a phone call from the FD. There was someone right outside my door with my laundry. I asked why they didn't knock, and they told me they were not allowed to if there was a DND sign on the door.

Different hotels, different management, different priorities. I only stay at College Park when I have to now.