I knew it was only a matter of time. I had to buy a scanner because it was 6AM and only Walmart was open. Turns out the engine had Dr Pepper instead of oil and Pringles cans in place of the pistons.
Nah, just a broken gas cap. I took it off and put it back twice and no dice, so definitely not just loose. Crazy that it didn't happen until after I burned through half a tank. I wonder what let it go so long before failing.