Are you the only one who can smell it? It might help to bring in an independent nose, trusted friend or family member with a reliable sniffer to confirm. (I smell ghosts, also)

I don't think a litter box smell and a dead critter smell are anything alike. Maybe someone else could pin down the "type" of odor.

I'm chuckling at your misunderstanding with your mother, and picturing a big snail crawling around in your attic. My brother lives in the south, and he would say "smail" for "smell" and I probably would have thought he said "snail", too. Hee hee.