hmmmmmm...
This might be more serious than it first appears. If I remember correctly, beryllium is commonly (or was commonly) used in nuclear weapons. I think I read that if a beryllium atom is bombarded with an alpha particle, the nucleus of the beryllium will become unstable and emit a neutron. If this is done on a large enough scale, enough neutrons could be emmitted to cause a chunk of uranium or plutonium to fission and possibly go supercritical. If this is linked to terrorism, it sounds like someone is building a nuke...
However, only beryllium-9 will emit neutrons when bombarded with alpha particles, so hopefully they stole the wrong type of beryllium 🙂