Marco, that's some great info you have included there. Very thorough, detailed, and complete. I think this is one of the most useful problem descriptions I have seen, because you have noted everything that happened during each of several occurrences.
I don't have an answer for you, but I sincerely hope somebody here will have an idea.
I am also a bit surprised that your service guy didn't find any error codes stored in the computer. I wonder, did they even check it? If not, I would insist that they do so the next time, even if you have to stand there while they plug it in!
Also, when your wife came out of the 7-Eleven, had she just filled up with gas, or was she simply stopping in for something from the store? The reason I ask is because my BMW started the engine warning thing, and the dealer didn't want to test it. They said they would have to charge me if it turned out to be a loose gas cap.
They nearly fooled me, too. I had decided to drive it another day or two to see if it happened again, and I drove away from the dealership. I got 3 miles down the interstate when the light came back on! I called my service guy right from my cell phone, and told him I'm bringing the car right back to him for full diagnostics. He agreed, and after they tested it, they discovered it was a bad O2 sensor or something.
My point is that service people (from all dealerships) typically like to say it's the gas cap, but unless they actually check the codes or test the car, they are really just guessing.
Good luck. I hope they find it and fix it very quickly for you.