I guess that could be a possibility. Try switching to stainless steel or a crock type bowl.
It still does very much look like he has been pushing something with his nose. Some dogs like to arrange their bedding and scrape their noses in the process...or, as I said, push food bowls around...or balls...or anything that may cause their noses to scrape the ground. Now that I think about it, I've seen that happen on concrete as well as carpet.
Maybe he is doing something when you aren't home. You could try setting up a video camera to see what he's doing.
Pam