I think those two oil lines into the thermostat gasket have o-rings. Maybe those lines are what is leaking. An extension mirror would help you locate it. I just took my thermostat gasket off last night. Some UV dye might help you find it to. I wouldn't just walk in and say they messed up the job. If it has only been a week since they worked on it I would think they would check over there work and identify the leak.