Good old Heimdall is a secret-knock deadbolt device that attaches to a door with magnets and turns the deadbolt handle with a motor. This project is from a few years ago.
It's a really basic Arduino controlled device that uses a piezo to sense knocks from the other side of the door. If the timing of the knocks matches a pre-recorded sequence –the secret knock– a hobby servo turns the deadbolt handle.
While the basic code for such a project is easily found on the Internets, Heimdall is unique in its hardware design. Twenty-four tiny rare-earth magnets (actually quite common) hold the device to a [fire-rated, steel] door over top of the existing deadbolt. The deadbolt can still be turned from the outside with a key.
Three arch-shaped bulkheads designed in Lightwave and printed in ABS on a Makerbot Thing-o-Matic hold two ½-inch square aluminium chassis rails. A single arcade button controls the knock recording function and manual lock operation.
Were I to do this project again, it would be controlled by a Raspberry Pi with a web interface and a touchscreen.