I agree with Minder. It looks like an electro-mechanical buzzer, probably an ad hoc door-bell arrangement, probably low-voltage AC operated from a door-bell transformer wired to a push-button switch mounted outside, near the door. Or maybe operated from a 6 V lantern battery. Follow the zip-cord to see what is (or was) attached to the other end. If you own the warehouse, you can safely remove it and then Install a wireless door-bell if you expect visitors.