You are a very important member of the radiation protection team at the nation’s border. You found a thick lead box containing some radioactive material. You can detect some gamma radiation coming out, so you know the box is “hot” (i.e. has radioactive stuff inside). So you put on your radiation suit and enter air-tight enclosure to investigate the contents. What is really strange is that when you open the box, you find it EMPTY! In fact, your weight measurements show the box to be about a pound lighter and your swab-sample from inside of the box show traces of Polonium ! 

Can you explain (with reasoning) what could have been in the box?