Somewhere in Utah on the way back from Expo I noticed that the oil light came not not long after we got going again after filling up at a gas station.
Turns out that I forgot to put the oil cap back on after topping up. I felt so dumb. I looked all around the engine bay but couldn't find the cap. Anyway, a couple more quarts went in and I duct taped the hole. It wasn't until I got home that I was able to find the cap wedged in way below under the battery.