Tailgate opened/unlocked while parked during rain and light snow. The tonneau cover disguised that the tailgate was open (held it up just enough so it was not visable). The tailgate fell open while driving but went unnoticed. Several large and expensive items were lost out of the back of the truck which is used as a work truck for electrical services. This has happened twice on the roads. Now the tailgate is physically checked before driving to ensure it will not fall open. The incident was mentioned to the local GM dealer but we were told our model was not part of the recall. Our next call will be to GM directly to try to remedy the problem and hopefully recoup the items lost.