More detail than you needed:
My last trip (10 nights, ASMo) I left the DND sign on most of the time. I was traveling solo, and didn't need someone servicing the room every day.
Day 1: Bag of towels, wash cloths, toiletries, etc. hung on the outside of my door.
Day 2: Another bag left hanging on my door.
At this point I have twelve sets of towels and wash cloths -for one person. I am being overrun! I call housekeeping and tell them I don't need anything else and to please stop leaving them. I am told they will cancel housekeeping until I call them back.
About half way through the trip I call to have the room serviced the next day. I take the DND sign off the door. Room serviced as I requested. That evening I replace the DND on the door.
Next day : Bag of towels, wash cloths, toiletries, etc. hung on the outside of my door.
I call housekeeping again. I tell them I don't need any more bags left on my door. I have too many towels! Somehow that message never got relayed, as they left bags every day after that until I checked out.
So in my experience, if you leave the DND sign on your door, they won't enter, and will leave a bag of towels and toiletries. That's what I've read here from many people too. I would think if they didn't leave a bag it is the exception rather than the rule.