You ran the engine for 30 minutes. If that was largely at idle, there wouldn't have been a lot of circulation nor pressure. It is quite possible for some air to come out once the thermostat had cycled a few times but I cannot give a definitive answer. Nobody can. I can offer some ideas though.
Driving slowly the needle returned to normal. You have to mark the level cold, Take the car, warm it up, give it a brisk run and if the gauge goes up, expect HGF. If it doesn't, park it up where it was when you marked the tank and come back tomorrow. If the level is the same then you haven't got a problem.