I would assume the mechanic would just put a pressure tester on the rad cap opening and pump it up to 20psi then leave it for a few minutes, pretty simple way to see if there is a leak.
"Looking" for a leak is not the best method unless you have a pool of coolant on the ground already, then you are just looking for the source, no "if" involved.
You can rent a cooling system pressure tester yourself if you want, not expensive and easy to use.
I would say, "if" there is no leak then you are smelling the hot coolant in the overflow tank, it isn't a sealed container.
On my older 4.0l, the overflow hose ran across the top of the rad.
This hose got a crack in it.
The overflow system isn't active until engine and radiator warms up.
In my case the hot coolant would exit radiator thru the hose and some would leak out on the top of the hot rad and evaporate pretty quick, so no sign of a leak but did have the smell now and then.
Pressure tested system and it was tight, no leak!
Started checking rad level cold, every morning.
It was always a bit low, we are talking 1/4", hardly anything, but not at the very top where it should be with this type of overflow system.
So I finally checked the overflow hose, found the crack, I just taped it, 6 years later still working fine.
Last edited by RonD; 01-30-2015 at 11:23 AM.