I had the exact same problem. We bought a house and I noticed during the winter the heater would run 16 hours a day. I have no attic access, so when I had it reroofed last spring I took a look. There was "insualtion dust" about 1/2" thick, but that was it. I ended up cutting a hole in the closet ceiling so I could get to the attic, and found the mummified remains of a squirrel. I don't know where they took it all, but it was gone.

I ended up paying someone to blow fiberglass in. My gas bill is less than 1/2 of what it was last winter.