▲▲▲ This and really isn't anything to worry about, thou one morning before you start up, remove the radiator cap and gently squeeze the top flow hose and check your coolant reservoir. The antifreeze should be right at the top when squeezing. What is happening is that the fluid is part of the radiator heating system.. and the bubble are trapped within the hose that leads tot he heater core unit under the dash.
Note: Make sure when you replace the cap, you mount or seat flush and turn the cap twice to lock in place. If this issue continues I would suggest that you get the Stant pressure release cap.