The local news here implied he may not have been drug tested for several years and had just slipped through the cracks. Sounds like he was in the system more than once, did not use his SS# and may not have actually been tested for drugs in several years.

Another Article

I found the last paragraph interesting:

"The updated database lists Halye’s last medical certification as September 2016. While a urine sample is taken during the tests, the sample is tested for diseases but not drug use"