the remote locking systems on these cars do have what is known as global open and some, dependent upon year and market version, have global close too.
Global open = press and hold the unlock button and all the windows drop.
Global close = press and hold the lock button and all windows close.
Honestly, I don't know if the sunroof is included in these protocols simply as I don't have a sunroof and never tried it, although that is what the dealer is referring to.
It is hard to know exactly why it would be opening. The car could genuinely be performing a global open type function, in which case you need to determine if there is any environmental factors which could have made the car think it was receiving a signal to do so - so think about if the car was in the same place when it happened, if certain events occurred around the same time it happen. virtually impossible.
The only other explanation is some kind of wiring drama. The cars are CANBUS so the sunroof is likely part of a body computer or something which will be requesting it open, so if you've got some diag you could take a took at it. again virtually impossible to trace it down.
If it gets to be a real problem it might be worth popping the control panel and seeing if you can spot the wiring plug to it, or dropping the headlining so you can access and either give it a good clean up and see if that helps or disconnect it so it can't happen. It might be a long shot but there could be some moisture or corrosion got into the switches / electronics surrounding the roof and causing issues as circuit boards can be exposed rather than sealed units so look for white or brown marks / staining on the electronics.
I know your pain in having weird electrical issues though. Every now and again when I pull the switch to have my window up, it takes the window all the way down and stays there and you have to wait 30 seconds or so before you can try the switch again to make it come up, if it feels like it. Better still is any time the temperature in England gets about 20C the windscreen wipers become very upset and can start getting bonkers, even when completely turned off and bone dry.