Fun story: I tested this bug, initiating a call from my phone and then joining it on my Mac. After I had ended the call, the Mac's camera LED stayed on, even though the FaceTime app was not showing a video preview (in fact it had no windows open). Was it transmitting? Who knows! State management seems to be a mess all over the place.
This means your camera was in use by some application. Whether that video left your device is harder to ascertain, but you can try quitting FaceTime to see if the light turns off.
