I was thinking something like that. I would LEAN (I'm so punny) toward a vacuum leak, such as a line off, a PCV fresh air line, etc. I am VERY curious to find out what it ACTUALLY ends up being.
I fixed a Cadillac yesterday with a PCM reprogram for a couple lean codes, with a couple others as well. It was on a 2005 with over 150k miles, and I found it interesting that it had never had it done! Ya never know!