Perhaps your webcam turns on when starting a guest VM. (thread 1, thread 2, thread 3, none with helpful info. There's also at least one bug for it, only two months old but still silent at time of writing.)
It turns out that this is related to audio. If I have a VM with the audio disabled—such as one provisioned through vagrant—then it doesn't turn on the webcam. I conclude that VBox is actually attempting to use the camera's microphone as audio input.
(I'd report this on the bug, if they had OpenID or something. But building a whole Oracle Account just for this? Meh.)