Can't see it clearly. If that wasn't on there when you bought the car, it has been tagged for some sort of ritual. If you wait around long enough you'll find out. Remove it if you don't like it and if it comes back, you'll know something is going on. Potentially an X-File. (There is an old TV show mental dig that came to mind. )
The obvious answer is that this car was once used by the MIB. That is their signature calling card to let civilians know beware and approach with caution, DUH! If I were you, I would start to look around the cabin or trunk area for hidden compartments that might still hold advanced weaponry from the agents. Who knows, you might uncover some hidden toggle switches that when flipped, send you into warp speeds. I call dibs on anything you find, as I am the only person here that provided you with an accurate solution to the origins of that decal.
not related but my true story is similar in that sense, in Vietnam we have a lot of motorbikes (millions). I never noticed that until a friend of mine told me your bike was marked. He showed me 3 black paint dot on the number plate and told me some original OEM part were swapped with fake ones. And he was right. He told me when bikes were parked in some area (paid parking) bad guys would swap original parts with cheap fake ones and left black paint dot on number plate so they time they know what was swapped.