My over flow was going down and I didn't see anything on the ground either. By chance I saw a stain by the O2 sensor and I followed it up to the evaporator drain.

A.J.

 

if im correct and im not 100% sure but when my heater core messed up it caused a whole mess of antifreeze in the passenger floorboard. i looked on other post about this and people were saying it was the heater core. also b4 i noticed the wet floorboard i had a faint smell of antifreeze

 

I wouldn't use stop leak. It causes more problems than it cures.
If I ever get a small leak in my radiator and I need to get it home, I just crack an egg in the radiator. it seals the hole temporarily, but flushes out a lot easier than that liquid weld stuff.

Coolant on the passenger floorboard and the sweet smell of it inside the cab sounds like a heater core.
If it's still overheating, your water pump might be bad.
if it only overheats at low idle, it might be your fan clutch.

Also, I learned this the hard way..there's a slight chance that using a pressure sprayer to fill the radiator can add air bubbles that don't get bled out which can cause overheating.