Usually someone uses it for a step to get into the rig. What year model is this? Let Parts know and order a new cap tomorrow. Next time this occurs, take a pop (soda) can, and trim it or tear it in half. Then take black tape and tape it on the hub itself. You can even pack a bunch of grease in the can for safety on the ride. You will have to repack the bearings on that wheel and replace the rear seal. To get the replacement cap to hold again, take a chisel and tap the inside of the new cap in Six (6) or eight ( places making star like notches on the tapered lip.These will grip the sides of the hub, and also give it a slight size change that will make it hold better. We dont have a way to press them in on replacement like the factory did. We are curious on the year model though? And parts will need that to figure out the model of part. Old hubs were DDR in the early 90s, while later ones are Unique, and there are different Unique ends. BCB