Haha, I have a 2015 Opel Astra (I think they are sold as Vauxhall as well in some countries) and I noticed a similar thing one day: it suddenly had all windows lowered by itself, without me doing anything to cause it.
The first time it happened was on a music festival after hauling a lot of camping gear from the car. I locked it using the remote key fob, put the keys into my pocket and hauled the last bunch of stuff to our camp. An hour or so later someone told me that my car had completely open windows, asking whether that was intentional. Of course it wasn't.
The next time it happened was at home, after shopping for groceries. I locked the doors, carried the box with the groceries into my flat, and when I finished unpacking them I looked out of the window and saw my car in the parking lot - with fully lowered windows. I thought it was a glitch in the firmware or whatever.
A few days later the same thing happened again - I shopped groceries, carried them in, looked out of the window - car had lowered the windows entirely by itself. The same glitch twice within a few days? In almost the same situation? How big are the chances for that?
Then it suddenly dawned on me.
I have quite a lot of stuff in my pockets. Including the car key with the remote buttons. Whenever carrying heavy stuff, boxes and such, there is quite a good chance of me accidentally pressing the "unlock" button not shortly, but for a few seconds. So I took my key fob, stood in front of the car, held down the button...and after five seconds of waiting, all of the windows lowered for just a little bit, and after waiting a few additional seconds they lowered completely.
Since then I know about an interesting feature of my car: it can remotely lower the windows for ventilation in summer.