A mobile storage system includes a number of storage units supported by a supporting surface, including first and second storage units that are movable toward and away from each other to create an aisle. A sensing system detects the presence of persons or objects in the aisle and the direction of movement of persons or objects into or out of the aisle. The sensing system includes exit zone sensors located adjacent an end area of the storage units that defines the end area of the aisle, and inner sensors located along an inner area of the storage units inwardly of the end area. The exit zone and inner sensors are located at generally the same elevation above the supporting surface. Each of the storage units includes a carriage that is movable relative to the supporting surface, and the sensors are mounted to the carriage via a sensor housing interconnected with the carriage.