Knock to open
‘A friend of mine happened to live in a students’ flat where such a thing happened: the landlord basically made them cram a bunch of stuff, which usually was in the main hallway, into a room that was too small for it, while in the same flat three rooms were completely empty, but locked. Luckily, the guy could lockpick his way into a room, but he asked me for a way to be able to re-lock it without the key and without anything suspicious being visible from the outside.
After some brainstorming, we came up with a James Bond-worthy concept: knock on the door in a certain fashion, and the door’ll open automatically. The idea was perfect: no need to drill holes, sneaky enough so the landlord wouldn’t accidentally trigger it, easy enough to remember.’
