Truth be told I wouldn't be concerned, unless the coolant level is not between MIN and MAX and you are overheating. You state your wife said the car was running hot, but you never mentioned where the needle was reading. Now you are saying the car never goes over halfway, so is it still overheating or not? Also when you fill the coolant tank, you shouldn't start the engine or open the bleeder. It sounds to me like you have a lot of air in the system and as the air leaves, your coolant level is dropping. I would fill the coolant to MAX, run the car until normal operating temp, shut off and let cool. After that check the coolant level and fill to MAX if needed then repeat. It may take all day or several days, depending on how you work. I wish you luck.