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.