This snail has been hanging out in my compost bin. I don't mind him there. I figured he was eating some of the decomposing kitchen waste. Or maybe he was just hanging out.
It turns out, he has been eating the brown stuff from the inside of the composter lid. Over the years, the lid has alternately laid upturned with water and gunk sitting in it, and then dried out. The resulting residue must be a crusty delicacy for a snail.
His trail clearly shows everywhere his "teeth" (radula) scraped on the lid, and his head moved from side to side as he grazed his way along.
Very interesting, and much more educational than seeing my vegetable plants with gaping, slime-covered holes.