Erratic mouse movements on 4 pc's at small business, security concern or wireless interference?

I have 4 PC's close together (within 10 feet or so) where users are claiming that the mouses are moving on their own. I witnessed it on 2 machines. They all use wireless mouses. They are all on a network, but one has no DNS info so it can't connect to the internet. The employees are concerned that it might be some kind of hijack or hacking. They have a Fortis firewall and supposedly they don't see anything. This all started a few days ago. In these few days, an outside company is doing an audit, on the floor above them and they are using a wireless mouse. I thought that might be causing interference. Sometimes the mouse movements seem purposeful, but most the time it is random clicks on the desktop, opening icon properties etc. We scanned for Viruses and spyware and the machines came clean. My question is has anyone experienced this? Is this more likely to be a simple interference type issue or something more sinister? Do people actually try to hack into systems allowing the cursor movements to be seen the whole time? These machines don't even store sensitive data locally; anything interesting would be on their server accessed through a browser or proprietary software running of a server. Wouldn’t they access the server? Can someone from outside the building get to these through the internet, even the one that doesn't get out because of the missing DNS info? That might be a dumb question, but I don't know how that works. I am just trying to help them out. I wanted to switch out to wired mouses, but they didn't have any and this has been very intermittent and it may take hour or days to see a difference. All PC's are XP Pro or XP Home.