There is a fault with the new model passat. where during the night the windows can come down and the boot lid opens, if it is a saloon anyway. It is due to an electrical fault or if he accidentally pressed the boot open button on the remote. My advice is taking it back to a dealer to be checked it is a known fault with volkswagen technical.
The only other possible answer is if it is a saloon, check the boot lid for damaged paint at the bottom of the rear screen in the middle where it joins the boot lid, as it is possible to stick a piece of wire under the boot seal and unhook the boot release handle on the top of the boot lid.