Eventually, I wandered over to the shower house to clean up. When I returned to the site, there was evidence that I had an intruder! my bag of granola was torn, and what was left of it was scattered around the picnic table.
Cleaning up, I saw scat left behind from the perpetrators. Once I got home, I checked my field guides and am pretty certain that my visitors were a buzzard and a goose. It's amazing that they hovered around and waited until I left the site, then dove in for the goodies.