That's so odd. I am sorry for your loss. We have a feral cat out at our pastures and machine barn. For almost 10 years she had litter after litter. We couldn't catch her, but we fed her and cared for her. We actually experienced the same thing as you are talking about multiple times. But, it was because she would move them and forget where she moved them to. My poor cousin (who lives out at our pastures) would spend days ripping apart her garages (she had two), the barns (there are three of them, two smaller ones and one huge one), and walking through the probably 10 acres of woods looking for them. Sometimes she would find them, and sometimes she wouldn't. But, there were times she wouldn't find them and months later another cat would turn up about the right age. We finally were able to spay the mama cat, and the other cats that were around were all male. But, they have vanished so I'm guessing they either died or left for new territory.
The mom of: Too many to list now.