I've been noticing something strange on the rear bumpers of various cars lately. Four raised or recessed circles, evenly spaced across its length. They seem to be made of the same plastic as the bumper itself, and I have no idea what they are or what their purpose may be. I like to think I have a pretty comprehensive knowledge of all things automotive, having been a mechanic for a few years and general gear head for many more, but this has me stumped. Are they plugs for some sort of mounting system for attaching something to the bumper, maybe? That is the best I can come up with, and it makes no sense. I've seen them on several different makes and models, so they're not exclusive to any one car or car company. Perhaps a poorly concealed part of how the bumper is attached to the car? No, that's just dumb. Any body else have any ideas? It's not an earth-shattering quandary, but it's bugging me.