Coolant low but fills up when car off

[Qsell]

As stated in thread title. Weird one. coolant tank low when I’m driving so warning light comes on but fills up when car turned off. I’m guessing air bubble but how would that happen? Could it be head gasket?
Quote

 

[Boydie]

Open cap
Let air out
Top up with water
Monitor

 

[Qsell]

Updating this:

Weird symptoms as I’ve had A LOT of golf r and s3’s which never had the same symptoms as above but this problem was fixed with water pump change.