Methods and apparatus are provided to monitor items on a storage unit. Some embodiments provide a system for monitoring items held on a storage unit, comprising: a scanner device configured to capture a 3D model of the storage unit; a memory configured to store a baseline 3D model of the storage unit in a predetermined stocked state and a plurality of 3D item models, each 3D item model corresponding to an item intended to be held on the storage unit; a difference extraction unit configured to compare the captured 3D model to the baseline 3D model and generate 3D difference data corresponding to a difference between the captured and baseline 3D models, and a product identification unit configured to identify items present in the 3D difference data based on the stored plurality of 3D item models.