I locked the car as normal but on my return found the car unlocked with all the windows fully down and the boot and sunroof open. How could this happen?

I have come across this before where the key fob has been held down on the unlock button

Its sometimes a reaction to see if the car has locked when you have walked away from it and as it auto-locks then its always uncertain so the tendency is to press the buttons to look for telltale indicator flashes and/or the buttons were inadvertently pressed while in the trouser pocket.
Not saying this is the case of your situation but it happened to my father in law who was adamant it wasn't his doing so a special wallet was made for the fob to drop the keys into when walking away from the car and never had an issue since

Thanks Trevor I did have the fob in my trousers pocket at the time so that is probably what happened. I’ll have to get a wallet like your father in law.