Hideouts can't be protected with a simple lock and key; those jobs require passwords or secret knocks. For the latter, you can go the extra mile and build a knock detector using a few basic pieces of equipment.
Steve Hoefer's secret knock detector is little more than a piezo speaker, a tiny gear reduction motor, an Arduino and some PVC pipe — basically a bunch of stuff he had lying around the lab. It works by keeping track of the time between a pattern of successive knocks. It's probably not the most secure way to lock a door, but Hoefer notes that there is a lot of room for improvement on this design. Besides making it small enough to actually fit in a doorknob, you could program it to accept a variety of knock patterns, including different knocks at different times of the day. Hit the link for instructions on how to build one yourself. [Grathio via Make]