I have added coolant 4 times (filled overflow resevoir) in the past year. It has a 165,000 miles. I bought it new. It is in excellent condition. Vast majority of miles are highway. It has never and still does not leak a drop of any fluid. I am worried about the head gasket. There is no colored smoke from exhaust. Runs great. How can I tell if I have a big problem beginning to show itself? Thank you so much!
How much have you added each time? How low was the overflow bottle - empty? Or a little below minimum?
Try replacing the radiator cap.
If the radiator cap isn’t capable of holding the proper pressure, coolant can escape from the cooling system as a vapor.
Take off the oil refill cap and inspect the inside. Is there fluid that looks like a latte? If so, you have head gasket damage letting coolant into the oil system.
Coolant in the transmission due to radiator internal leak?