Doctor Who, and the Doctor Who: Adventures In Time And Space roleplaying game. By Craig Oxbrow.
Hmm, two possibilities spring to mind. Time travellers, who plan to use it as a base for their nefarious activities, and have moved it into the past where it won't easily be noticed. Or a group from a parallel universe who've jumped it home or to some other Earth. I believe the latter was done by Infinity Unlimited, though they stole a station that's been abandoned after nuclear war broke out on that Earth.
Was this meant for the stolen space station post? Either way, good point, mysterious visitor! :)