This question already has an answer here:
The Black Hat conference (I wasn't there) had a session on "ratters" (RAT for Remote Access Trojan) who take illicit footage of users (usually female) by hijacking their systems.
The question I have is if this happens on a system or web-cam device that has an "in use" LED on it, is it possible for them to do this without turning on that LED? On my MacBook for example I would definitely notice the LED. Many USB cameras have the same thing.