I think that the auto-rescan is a very BAD idea. It would severly slow everything down and make the network more congested than it already is.

Just think of the massive effects if everyone on the network was periodically rescanning all their files. This would affect packet transfer rates, which in turn would affect the entire network.

Nothing can really be done about those people who move their files, especially if they do it intentionally. If it's a matter of forgetting about them, than maybe a simple timed reminder window is in order, something that pops up and automatically disappears after a short time(15-30min intervals)