Wireless mouse interference, or...?

I have about 20 users of XP Pro PCs on an regular Windows domain. Today 4 users called up at the same time to tell me their cursor was moving around the screen on its own; I've seen problems like this before with optical mice, but I knew something was different this time because when the mouse was moving it was very purposeful instead of some random movements.

So I checked the firewalls to make sure no one was making any suspect inbound connections, I virus/malware scanned the PCs, I also used rootkit-revealer to find out if anything was on them, but everything came back negative.

What's also strange is, all the cursors move in identical ways, if one's moving left they all move left.

This is happening all the time, about once every 15-30 minutes and only very briefly, for maybe 30 seconds at the most.

Now, I was thinking perhaps it's a hacker, but why alert the user by controlling the mouse? Why not prevent me from running the virus scans etc..? Plus there's no inbound or outbound connections to these PCs according to the firewall. This just doesn't add up.

So I was thinking maybe, as silly as it sounds, as they all have the same make and model of wireless mouse perhaps one of the mice is malfunctioning and not jumping frequencies as it's supposed to if it gets interference on the same channel? I did unplug the wireless receiver from one, it stopped, but none of the other PCs experienced the problem either.

Any other ideas as to what might cause this?