Hi, welcome to jA.
When the car overheated and you saw the coolang gushing out - did it not come out of the overflow bottle? When the car overheats the coolant overflows...are you loosing a small amount of coolant over a long time period as well?
It must be a leak that does not show when the system is under pressure...it must leak as the engine cools slightly...seems like all the standard basis have been covered. Coolant can leak from nuemerous points on this engine, the best step to take next is have flourecent dye added to the cooling system. Drive the vehicle untill you need to fill the resevoir next...than take it back to the shop and have them use an ultraviolet dye to locate the EXACT point of leakage. The dye will mark the area the coolant leaked out and using the light will verify this. You can even purchase your own coolant dyes and lights if you want to attempt this yourself, but if the leak is somewhere difficult to see with the car on the ground, you'll need access to a vehicle lift.