Heroshima - An Attention Hazard

In a way, using the on japan, the isotopic signature told everyone what the americans had managed to do. So without the blueprints of the bomb, they could look at the radioactive signature of heroshima and get a lot of hints. The big one is, as you said, an attention hazard. It says, this is a weapon powerful enough to destroy cities, that could determine the future course of warfare. That's a giant neon blinking sign in the sky for any one who wants to acquire power.