It would be slighly more useful to have something that uses the microphone to detect when you physicially tap the laptop e.g. with your finger, it could be used

It would be slighly more useful to have something that uses the microphone to detect when you physicially tap the laptop e.g. with your finger, it could be used to keep typing even with your laptop screen down, imagine a spy movie where the baddies close your laptop and put a gun against your head and you have to put your hands in the air, but you use your knee under the table to tap type "shred -vzn 0 /dev/xxx", poof, all data gone.