Caroline & Mssrs Padfoot & Prongs
There weren't many ways I could put myself in real danger and be sure that I'd get back out of it again. That was the safest idea I had, believe it or not. It'll work while they're alive, if they're in danger. And when they're still fundamentally themselves, but not visible or in another form (for instance, as a dog or a stag), it'll still work, so at least you won't get any false grey readings. Just not sure if it'd show up as red or green. Depends on how dangerous it is to be that kind of vampire, probably.
I can imagine it'd be weird. I don't think I'll ever entirely understand how the telly works, though.