Coolant High Temp Flasing, Clears Then returns

A bit curious on this one. I was getting a random alert on the coolant temp. Would be solid, then clear. Later it would blink several times then clear but then come right back solid. with the speed around 50 mph give or take 5mph. Checked the coolant overflow tank and it was low. Added some purified water and then it cleared. Added 50/50 red Toyota coolant and filled tank to a bit over half. Seems to be doing ok but I checked the tank after overnight and it was low again. When I opened the cap there was a bit of pressure released. Topped off again and it is like the coolant is going somewhere, I can't figure out where. No leakage noted on the ground under the car, the oil is clear so no head gasket leak. Is it possible that the level in the engine was low enough that when the engine is cool there is some flow into the engine? I will check again tomorrow and add more if it is low. Just never had an issue with coolant in my prius. Any suggests on what to check for or am I missing something? Thaks