No-one in IT would have given out WEP codes, but possible that someone came into our room and picked up some paper with them on.
We're going to look into upgrading to WPA with complete new keys when we're not so busy.
In the mean time i've reserved the device an address and pointed it back on itself.
That should slow the person down while I get busy accessing the AP to block the address completely.
Many thanks for all your suggestions!