Flagging noteworthy moments during lengthy video recording?
If the camcorder records stereo sound and you are only interested in mono recordings, the foot switch could trigger an oscillator that injected a short tone on the left channel and the inverted tone on the right channel. 

During regular playback where you might be mixing left and right audio to one mono audio signal, the tone will be unheard because of electrical cancellation of the signals. However, if you had a small electronic circuit that looked for the presence of the tone on (say) the right channel then it would find it.

A better version would use a balanced differential amplifier to look at both left and right together - pure mono sounds (i.e. the sounds you collected in your recording) would be totally cancelled by the differential mixing in the amplifier whereas the "tone" you injected would be plainly and unambiguously heard reminding you to play back the regular recording at this point.