Radiator not staying full.
If you have fluid in your overflow tank AND your overflow line can maintain a vacuum/siphon AND you don't have any leaking coolant, this is probably your issue. (By process of elimination. If fluid is communicating back and forth between the overflow and the radiator and the system is sealed/not leaking, this is the only way air can get in to they system.)
If you don't top it off - that's not a problem. As the car heats up, the coolant will expand and start flowing back-and-forth to the overflow as normal. The problem comes when the amount of air in the system is greater than the amount of expansion.
Either way, this is your canary in the coal mine and you'll want to fix it before it leaves you overheating and stranded.

If you check the level of the coolant after you turn off the car and compare it to when the car is 100% cool, that will confirm what is going on. If you check the hot coolant, you'll want to put a heavy towel over the cap as you open it and keep it half-way turned until the pressure bleeds off before turning it the rest of the way and removing it. That stuff burns!
- You've lost suction between the radiator and the overflow tank. (Leaking hose between the two.)
- Your pump is bad and you'll need a new one.
If your hot level is high, you'll need to put a vacuum on the overflow line to make sure its good. If it is - you'll need a new pump.




