I have never lost my keys, but I imagine that all hell would break loose if I did. Seriously, losing your keys—especially if you live alone—would be a nightmare which is why knock-detecting door locks like this one are genius contraptions.
This Secret Knock-Detecting Door Lock by Steve Hoefer makes use of a personalized ‘secret knock.’ You simply thump out your secret knock and the sensor will recognize it (when you set it up it stores the timing and rhythm of your knock), and then unlock the door for you. Watch the video to see how knock-detecting door locks like this work (and maybe try building you own!).