Mysterious iMac appearing in my network? Hacker?

Guys,

So I have two iMac 27's and a MacBook Pro in the household.

All of them are on my network (both wired and wirless via an Apple Extreme)

Now if I open "Finder" and look under "Shared" then - all of the mac's I expect to see appear there - but also a mystery machine - something called "imac-4431be"

I cant connect to this Mac - and I don't know how it got there?? My house is about 400ft away from the nearest residence - so I guess I could be seeing someone else's iMac - but how did they get on my WPA protected network (and if they were clever enough to do so then I'd guess they be able to keep their machine hidden!)

I dont understand it? I don't know where this machine is?? It could be my Apple TV or iPad or something could it?? ...or one of my iMacs was a refurb - it couldn't be a residual machine still showing??

I'm lost!

Thanks!