Every once in a while I hear a loud sound from my Mac. It sounds like either a camera shutter or a keychain lock/unlock. It all started a couple of weeks ago.

I fear something is snapping shots of my screen and beaming them somewhere.

I've scoured Google and found others have experienced the same issue. Based on suggestions, I've installed 2 Anti Virus apps (one at a time, of course), scanned my computer, and found nothing; checked Activity Monitor for unknown processes - none I could identify; looked at all the regular places (~/Library/LaunchDaemons, ~/Library/LaunchAgents, /Library/LaunchAgents) - didn't see anything out of the ordinary.

What am I missing, what else should I try, or am I just overreacting and there's a simple answer?