I read a fic once where Dru kidnaps Buffy (may even turn her) but they are hidden, cloaked in a building but eventually Spike finds them and Dru has gone.

Angel may be looking with Spike for Buffy and they can sense her but just can't get in the building. When they do I think there is a table just sat in an empty room, Dru has gone. Buffy may have been bagging blood if she was turned in this, can't remember.

Evolution by Demonica Mills (definitely has the house bit)