There is a Siberian breeder in my home town, I've just e-mailed her with a couple of photos to say "I'm probably being paranoid, but just in case..."
I agree, that heart-wrenching though it would be, I would be inclined to leave a moggy with the new owners that had gone missing so long ago but found a happy home. However, if you had paid a couple of hundred quid for a pedigree kitten? ...
I couldn't not e-mail the breeder though - as you say, the worst thing when a cat goes missing is not knowing.
EDIT: thinking about it sensibly, most breeders microchip kittens don't they? And if a pedigree goes missing, most people would let just about everyone know - certainly the CPL would have been informed I would have thought. So Amber is probably just a moggy who coincidentally shares Siberian features - which suits me just fine, she's a poppet whatever she is