You should be able to easily identify if they really did replace the expansion tank. This is a white plastic tank about the size of a 1/2 gallon milk container attached to the side of the radiator. A new one will be white, an old one will be yellowed from age.
This leak could be a bad hose, or a bad seal on the level switch.
This page shows the diagrams and parts for your radiator area.
Here is one view of the diagram. The expansion tank is on the right, part #5. The level switch is #8. This is the switch that turns on the light on your dashboard: