There's certain times when the diagnostic trouble code system will run the test to see if a certain part of the car is working right. Like he said, usually EVAP operation occurs under deceleration at a certain speed.
It will go off by itself after a certain number of successful cycles of proper operation, but you can have the code cleared if you want, and then see if it comes back. If it's not vapor leaking from the fuel cap, it's going to be something else along the line of the EVAP system, such as the EVAP purge solenoid/canister, or the EVAP hose itself.
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