2002 nissan altima losing engine coolant please help!!!

[Pr0t3g3]

2002 nissan altima 2.5s
so a while ago i've been losing engine coolant i took it to a mechanic there is no visible leak out of anywhere he said it was probably the radiator cap or upper hose
i went last week to change those two and drain the fluid and re top it up and also put a new thermostat in because my old one was clogged which caused my car to almost overheat
we changed all that and then this week i noticed i'm losing coolant again strangely i have no idea what it could be
my mechanic said it's probably just the air bubbles still in the system hopefully that's what it is.
my head gasket is not blown if you're wondering my oil and engine are perfectly fine no mixture within them and no milky looking oil.
any ideas?