This cloak is of fine, if unremarkable dark cloth with excellent workmanship.
It will attempt to pickpocket strangers at random. It uses shadowy hands that emerge when nobody is looking. This chance increases when the chances of detection are low - pressing crowds or darkness. Any items collected will appear in one of the cloak’s pockets. Value of the items is not a concern to it.
The wearer, unless paying close attention, is likely to be as unaware of this thievery as its victims. An aware owner can will the hands to act.