A
surveillance device, most commonly a hidden
microphone or other concealed listening device (piezo sensors, for example), is also referred to as a
bug. This may stem from the concept of being
a fly on the wall, or the notion that an concealed listening device was like a bug, unwanted and hiding.
From Die.net:
: tap a telephone or telegraph wire to get information; "The
FBI was tapping the phone line of the suspected spy"; "Is
this hotel room bugged?" syn: wiretap, tap, intercept
http://dict.die.net/bug/