Last time I stayed at a hotel, the do not disturb sign was taken as more a suggestion than a requirement by the hotel staff. The DoorKnob condition, really weird name, takes the knob and actually retracts it into the door so the knob can’t be turned.
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From the picture here there appears to be a slot under the knob for a hotel key card. This is a mere concept from Yanko Design, but it looks pretty slick.
It could be useful in the dorm too if you are “busy” and don’t want someone walking in on you. I guess you could put t timer out there too so they know when to come back, but it might bruise your ego if you don’t use all your time. Via OhGizmo!