“Engine Hot A/C Off” dash light, engine not hot

Yesterday I drove my car about 10 minutes in 30 degree rainy weather, parked the car for about an hour, then proceeded to drive home. About 7 minutes into the drive the dash read “Engine Hot A/C Off”. I had the heater on at that point. I looked at the temperature gauge and it was all the way down on the cold side. I turned off the heater and continued about 4 more minutes and the warning went off on the dash and the temp gauge went up to about half way. I didn’t feel or hear any engine problems. The car hood was not even warm at all when I arrived home about a minute later. I checked the coolant this morning after it sat all night and it was just a tad under the cold fill line. I saw no obvious leaks, and nothing on my garage floor. There was some minor water condensation on the inside of the oil cap. Any ideas?