Phoebe, as long suspected, is a freak.
I took her to the vet for a major service a few weeks ago (manky ears, itchy rash) and had them look at a strange lump on her ribcage. I had noticed it about a month prior because it was bleeding. It didn't seem to bother her too much. They took a biopsy (she didn't like that at all) and couldn't really see anything except inflammation. They wondered if it would reveal itself to be a wart once the inflammation went down and she was given a shot of steroids.
She presents her belly to me on the couch and I give her a tickle. I start counting nipples. The lowest pair, then the next higher pair, then I find one half covered by fur below her rib cage. I look for its partner and.... nothing. I remember her mystery lump and check it out in its less inflammed state. Bingo.