This is a "hear hoofprints, think zebras" situation.
The overflow tank return is a siphon.
You need vacuum to run a siphon.
The little gap between the top of your antifreeze and the nipple for the hose to the overflow tank is the amount of vacuum you need for it to work, measured in inches of water column.
Resist all urges to check this for the period of time you've been checking it. Check it daily? Now check it weekly.
I bet you'll find the coolant in the same exact spot, and not have an issue, provided you've thoroughly burped the system.
The overflow tank return is a siphon.
You need vacuum to run a siphon.
The little gap between the top of your antifreeze and the nipple for the hose to the overflow tank is the amount of vacuum you need for it to work, measured in inches of water column.
Resist all urges to check this for the period of time you've been checking it. Check it daily? Now check it weekly.
I bet you'll find the coolant in the same exact spot, and not have an issue, provided you've thoroughly burped the system.