CEL on the highway making Wal-mart pitstop

[Perfectlap]

So of course this happens on the way back home of a long drive on a Sunday evening and the day after I took the OBD scanner out of the car. ...
the check engine light just came on. I pull over thinking I might have left the oil cap loose last night. I checked the dip stick and it seemed pretty dry. The not so accurate digital oil level gauge indicated one bar below so I added a 1/4 of a bottle of Syntec. Let it sit a bit tried the digital gauge again and it back to being on the money.
Fast forward 30 minutes ago and the oil cap is not loose. But... The coolant is way low about half of the minimum hash mark.
I'm about to run into the wal-mart and top it off. I recently had the tank flushed.

Suggestions before I pull a stupid? Leaking/cracked tank?