Most often those tiny annoying leaks are from a week clamp between the aluminum core and the plastic end tank there is a rubber gasket crimp between them.
Since the aluminum expands and contracts more than the plastic the Rubber seal moves around and can leak a small bit.
I have also seen them leak coolant from around the trans cooler fittings for the trans cooler seals against the inside of the plastic tank and those large locking nuts hold it in place from the outside.
the most common failure is a crack in the plastic tank below the upper radiator hose connection.
It seems the plastic doesn't take temperature and pressure changes too well
