HI,
That is normal behaviour because there is a drop in the airflow around the engine bay when you are not moving. It becomes more efficient when you move.
It does not really matter where you live because the cooling system is a closed (sealed) one. The only reason you would need to replace lost coolant is if there is a leak somewhere, and it sound as if you have a small one.
The ABV is your first thing to look at if you have aleady done the obvious of checking all hoses, joints and the radiator, including the heater one.
Do NOT whatever you do use anything like RadSeal or will probably cost you in the long run. Migs do not like Radiator sealants at all.
get the motor really hot and then stand it on a level clean surface and wait until it cools. I always cover the ground in white paper under the engine bay so any leak will leave a stain. Once it is cool, overnight is best, have a look under and look for the stain. The leak will be confirmed by any stain.
The ABV sits at the rear of the engine at the top, usually hidden by the plastic scuttle trim. The ABV is a cheap part and worth doing for peace of mind.
Good Luck
