This has happened to me 3 times.....although like you, the first time I went in to get groceries, and when I can back my tailgate was down......thinking the key fob button was pushed accidentally in my pocket......fortunately nothing was in the back. Second time, a bit more serious.......I was driving for over 100 miles on my trip, when I came to a near stop in heavy traffic on the expressway.....then when we resumed, another car flagged be down, and sure enough, my tailgate was down......and garbage was spread over the last mile......fortunately nothing of value and did not damage vehicles behind me. I was not so lucky with my truck......I had ice build up on the bumper, so when it went down, it dented by tailgate which will have to be addressed when the problem is resolved. Third time, after having it looked at by a local dealer, I left to go home, stopped for gas including 3- 5 gallon cans in the back......went a few miles, having the tailgate open.....and going around a roundabout intersections the 3 full cans of gas were spread out across the road.....fortunately no gas escaped and no one was injured. A concerned citizen tracked me down, and let me know the situation. I am so glad it wasn't my hunting rifles scattered down the road, and certainly am glad this hasn't caused any injuries or damage that I know about.
I don't understand why there is not any indication of the gate being closed......green light or red light (the gate is open)on the button....there seems to be a spot for it. Although there is a known issue, a gate ajar warning of some time is needed, this is a major liability for GM and for the owners of these vehicles.....I own a 2023 Sierra 1500 AT4.