I smelled the spit up before I actually saw it. After investigating, I found that he had sour-milk spit up pooled in the stroller tray. He was finger painting in it.
I scrambled to find a wipe or a napkin (I had, of course, left the diaper bag behind because we weren't going to be gone that long). By the time I found an already-used Dunkin Donuts napkin at the bottom of my purse, the mess was mostly gone.
...because Chase was licking it up. Ew. Ew. Ew!