If I truly do not want to be disturbed, I will call housekeeping ahead of time.

But at many properties when I have not given staff notice, I've been called about 1-2:00 pm and told that housekeeping staff is about to go home--last chance for me to have the room cleaned.

I guess many folks walk off for the day and forget to remove the DND sign.....more than those who intentionally leave it posted.