Staff discovered a quadrotor UAV atop the roof of Prime Minister Shinzo Abe’s office in Tokyo on 22 April 2015. The craft, emblazoned with a radioactive sign and with a minute amount of cesium found on it, raises the spectre of such craft being used for terrorist purposes.
It is unclear when the craft landed on the rooftop, but investigators believed it could have been there a day or two. A plastic bottle containing liquid with trace elements of radioactive cesium 134 and 137 was attached to the craft, though at levels too low to harm a human. The source