FIRMLY within the spirit of good prank hacks like what has just been commented: a small number of USB webcams' LEDs are user-controllable, such as the random old Logitech C300s I have floating around here. You want webcam-tools from SF.net (other versions will spout obscure errors), then you can do

  $ uvcdynctrl -s 'LED1 Mode' <0..3>

0 is off, 1 is on, 2 makes it flash (and you can even set the flash rate), and 3 is auto.

0, 1 and 2 apply regardless of whether an application has an active video stream coming out of the camera.

Shh. (Hi NSA)