This one was discovered by forum fellow Tom303 (http://www.1addicts.com/forums/showpost.php?p=12850245
): a hidden storage area under the trunk felt pad above the battery compartment.
Forum fellow Derek8819 pointed out that, on former 1ers, this space was used for installation of add-on modules (such as SATNAV and Bluetooth). In the 1M these devices, if chosen as option, are installed in the head unit. Hence, consider it as the fringe benefit of a secret pocket (which is eventually related to the 'drone sound' syndrome).