I've seen hotels that will have a notice telling you that they'll stop in to your room every other day or every third day even if you have a Do Not Disturb sign posted but that's just housekeeping and it's to make sure you aren't dead or totally trashing the place. That's clearly not what happened here. I'm in agreement with the people thinking that the OP either somehow accidently triggered interest from security or someone in another room did and security knocked on the wrong door and the only reason they even mentioned the Do Not Disturb sign was because it was an easy thing to bring up. If the sign hadn't been on the door they would have been there just the same but given another reason for the knock.