Just an update
Mechanic got busy, so I bought a scanner and got the code myself. IIRC P00455, large evap system leak. I reset the code, tightened the gas cap, and the code has not returned with ~2000 mi. of driving! Code reader has about paid for itself. I'm disappointed that the mechanic did not reset the code when we took it to him the first time to see if the problem was serious. He relayed to the wife that the problem was not serious and we should schedule an appointment for a later time. This is the first time that I've questioned his work in several years of doing business with him.