If your car is heating up and you cannot find a leak in the radiator, or the hoses and you have followed the correct procedure to eliminate air pockets in the system as well as diagnose whether fan is working. As well as checked for signs of milky substance under oil cap with not results.

theres a few more things.

Check radiator cap is holding pressure but usually cap doesn't fail,

My guess would be your car has a head gasket leak (not completely blow hence no white smoke coming from exhaust) and the leak only shows up when car gets hot which is driving heat into cooling system forcing the radiator cap open and pushing coolant out of the over flow bottle.

True story