Tell your dealer about it. The problem is the tow hook receiver boss sits too proud of the aluminum crash structure. When the bumper cover flexes due to wind loads at speed it presses the cap against the boss and pops it loose. It is a common problem according to my service manager. He said he sees it fairly often but nobody complains about it so he has nothing official to report up the chain of command. He suggested if enough people reported the problem they will issue a recall for it.
To be clear, mine has not fallen off (yet). The common failure mode is for the cap to begin to pop off and then the owner simply pushes it back in. If you do this enough times, the teeth that hold it in will break off and it will eventually be lost on the road.
Further disclaimer, my service manager is a car guy and we have talked about stuff like this for hours on end. While it is very easy for car dealers to make excuses, in this case I believe what he said.