Hi i have a standard 2006 mk focus st the other day it overheated suspected headgasket on the way

So today i have flushed the coolant system put some steel seal in refilled coolant did as it said on the bottle took it for a drive around 3 hours later everythin was goin fine temp was sittin normal then for around 5 seconds it rose close to overheating but then drooped back to normal.

Once returned home opend the bonnet whilst engine still running all the coolant had dissapeard turn engine off opened the coolant and it all came back.

I did make syre there was no air leak before driving and everythin looked fine is there something i am missing? Any advice is appreciated