It happened to me twice this morning on my 2013 Volt.
I think that is it! Just yesterday, I wanted to get the key fob out of my front pocket (don't like the bulk), so I started hanging it from a carabiner on my belt loop. (see image below) So I'll bet I'm sitting on it and that button is being pressed. I didn't even realize that there was such a button on the remote. :-(Is it possible the button on the remote got pressed by accident? If windows can be opened that way, I imagine the charge door could as well.
(Does the volt let you open it while in drive?)
Now that's funny! That puts a new spin on the dentist saying, "open wide!" LOL!I had accidental fobbage on time. I was in the dentist office lobby and looked out and noticed all my windows were down. I was sitting on it and hit/held the unlock button - which auto opens all windows.