unless you have an aftermarket alarm like total closure stuff that could have gone wrong then I would put it down to passenger error in that someone has caught the button and not really noticed.
i think the passenger side window is auto down so if the right set of circumstances got together its possible someone just caught the button and it will have gone down all on its own!
i had a similar issue in a brand new 270bhp audi TT-S I was driving around the docks off the boat. the boot release button is in the drivers door pocket, next to the fuel filler release. they keys are left in the drivers door pocket. so whilst jumping in and grabbing the keys i must have caught the button for the boot release.
so i jumped in a hoofed it across the terminal, they're chuffing fast these things. they're in trasnport mode and i didn't have seatbelt on round terminal so alarms going off all sorts I didnt see it fashing up on the dash until i braked and heard a bit of a thump and the rear window turned the same colour as the car, I thought aaaaahhhhhh....... boots open then! lol
unless it happens again I would write it off as a weirdo alfa moment. I am forever forgetting to lock my car and it wouldn't be the first time I've left the windows open either.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso