Um, I'm not sure. I guess they could have been alive a long time, but hiding out... Elvis?
Nah, that's my natural crack!brain talking... really I'm drawing a blank. Could be that there's vamps causing trouble somehow...building up a vamp army or something? Heh... probably better you choose.
Other possibilities if you're looking for some. I kinda enjoy prompting others.
If you wanted to cross TW... you know how at the end of Buffy they had all the slayer potentials become slayers... well they'd be in Britain too right (and all over the world), and maybe TW has caught wind of it one or more of them and is investigating. Bring on Giles... Might work. Maybe?
Mulder and Scully track down some vampires and find Giles has beat them to it?
DWxover - Giles hires Donna as his new secretary (post final S4 ep.) and he starts to notice some odd things about her.
Giles meets Mulder in a laundry mat... they're both covered in mud.
Dunno, like I said, my brain is almost always full o' crack, so you don't have to write any of them, but just in case anything takes your fancy... Good luck!