Just had the same message on our '06 Touring with no apparent cause after driving 800+ miles in two day. It came on several times in a row after several restarts, re-tightens, removal and replacements, etc. This was probably 2/3 of a tank into my 4th fill-up.
All I know was it had been quite hot (over 100 degrees) but this was in the evening, a few hours after the main drive, running to the store. It continued for a couple of days. The car had just seen larger temp shifts than the car usually sees, and much longer driving periods.
In any case, the car got light use-- maybe 50 miles over a week, then another 800+ miles yesterday. I figure either the cap WAS actually slightly loose, though it felt tight and just took several days to reset, or else it was a fluke caused by the extreme temp shifts or something actually breaking the seal that cleared out of the way. (A little piece of trash?)
Could be a bad cap, but it just passed emissions and it stopped the error for the next several fillups...
Strange...