I bought this gadget for my car some time ago, the PermaCap, which is a replacement for your gas gap. The selling point of the PermaCap is that you don't have to unscrew your gas cap to refuel--you just shove the pump nozzle through the spring loaded door, and pump away.
While it worked, it was great. After several months of use, the PermaCap caused my car to set an evaporative emissions DTC, and turned on my "Check Engine" light. After replacing the PermaCap with my stock gas gap, my code went away and did not return.
Permacap detail, showing the grungy flat rubber sealing ring on the fuel cap door
I suspect that what happened is that the rubber sealing ring on the cap door picked up some grime, which caused a microscopic leak. Today's OBD-II diagnostic systems are required to detect a 0.020" leak in your fuel system. Most of them use a pressure leak-down or bleed-up method: they apply a vacuum to the fuel tank by pumping the fuel vapor out, either with manifold vacuum or an electric pump; or they apply positive pressure with an electric pump.
Kleen Wheels Corp, the maker of the PermaCap, claims a lifetime warranty, so I will see if I can send the cap back for a replacement, and perhaps they will have some advice on how to avoid "Check Engine" lights in the future. I suspect they will tell me that I need to remove and clean the PermaCap once in a while--which would negate most of the value of having such a device to begin with.
I also see that on their web site, they are selling a "new and improved" PermaCap II--so maybe they fixed it?