Automobile Enthusiasts

Warning messages or lights show up if a control module(computer) in your car senses a problem when it performs a diagnostic test and sets a trouble code. The loose fuel cap message means that the engine computer detected a leak in the gas vapor containment (EVAP) system. Some tests are performed very frequently, others are not. EVAP system leak tests are not. The test might run once per drive cycle, IF multiple conditions are met. So if you are driving once a week, this test is running infrequently. The most common reason for a leak is a loose gas cap, but there can be many other reasons. If the cap was not loose, there could still be a leak, and the code may reset...eventually. If he didn't charge you, I doubt the Midas guy actually tested for leaks, which is the proper way to diagnose this concern.
If a problem such as a loose cap is fixed but no one erases the trouble code, the warning light will not disappear immediately. It should eventually go away if the computer runs several tests that pass. How quicky that happens depends on how frequently you drive. Auto parts stores can usually scan and erase trouble codes for no charge, though some state laws may prohibit them.