The Raw Story | Could missing Wal-Mart signs wind up as dirty bomb? 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.