* A random concerned citizen (so it wasn't them looking, they just acted on behalf of someone genuinely concerned, this makes the thing a bit more noble).
* A scorned lover/an enemy ("oh you think are going to go on a vacation? remember what you did to me, I know exactly how to get back... what is that FBI tip-off number again...?")
* A secret (or not so secret govt.) web searching and filtering program. They don't want to reveal which one it is, but it could potentially just be made to generate "tip-offs" so that it looks like a person noticed, but in fact everything is automated. High profile tip-offs can be filtered through human agents as well.
Some of us who lived repressive regimes know how this works. If the govt. is afraid of X, and sets up an anonymous tip-off line to report X. Then is known to go ahead and blindly act on that tip. It creates an awesome/terrifying tool for everyone to use. X can be anything you like: terrorism, communism, whatever the du jour "War on ..." is waged.
In the Soviet Union we had neighbors denouncing each other for anti-communist activities because they couldn't agree on the color of the fence. This stuff will happen. The crazier and irrational the govt. gets the more potential for abuse it creates. With a bit of work and ingenuity, during certain decades, you could have made your whole neighbor's family disappear into Siberia practically overnight.