A little over a year ago, a routine audit at Wal-Mart reported a few missing exit signs at the company's stores and warehouses. As the audit continued, more and more signs turned up missing, and a month ago, Wal-Mart revealed that as many as 20% of the 70,000 signs at its 4500 facilities cannot be accounted for, a stunning total of 15,800 signs in all.
This would be of no particular concern -- except that the signs are radioactive. They contain tritium gas, a form of hydrogen which is used for emergency exit signs because of its ability to glow in the dark when the power goes out.
The signs are being stolen (probably by underpaid Wal-Mart employees) because Tritium is valuable and cannot be traced. The same glow-in-the-dark ability used in the exit signs makes it popular for gun sights, and other night-time powerless illumination devices.
The claim that this is all a dirty bomb plot is a bit silly when you realize that the instant tritium is released, it floats upward
(think Hindenburg) away from the ground and away from people. Yes, it will eventually combine with free oxygen to form water, but by that time it will have become so dilute as to be undetectable against background radiation.