sounds like a vac leak to me

This afternoon I looked under the hood, but everything looked "brand new" to my eyes. I still change my own oil and filters, but otherwise I don't mess with the mechanics. Except for the coolant hoses, all lines appear to be of a hard plastic, so my impression would be that either they are A-OK or would have to be broken off to be leaking. These vacuum hoses are different that those of yore which were of the pliable rubber type and more prone to wearing out and leaking, I would think. Perhaps I'm being too cynical, but my feeling is that the garage can't find a leak, so does the easy fix and replaces the gas cap. Everything is fine until the light comes back on . . .

A vacuum leak is hard to find with the eye'*. get a can of carb cleaner and while the car is running, spray near a few hoses. If the car rev'* up when you sprayed, you've just found the leak. Repair, try again and move on.