Methods, systems, and machine readable medium are provided for a detection system for identifying unsafe activity occurring at a shelving unit. A plurality of distance sensors are disposed at a top end of the shelving unit and point downwards towards a bottom end of the shelving unit. The plurality of sensors are configured to detect presence of a person near the shelving unit. An initial height of the person is determined, and a change in height of the person is detected. A processor determined whether the change in height indicates unsafe activity at the shelving unit. If unsafe activity is determined, then an alert is generated.