It may have just been electrical interferance.

But electrical interfance from a trackpad? - made me suspicious at the time.

And one way or another, a proof of concept is only a few lines of javascript.

A very very easy way to link people in a room together.

And it would explain why the facebook app wants mic permissions when afaik it offers no audio features. it doesn't even need to be audio the maker of the app controls - just "fingerprint" sounds and report them back to facebook (or whoever).