Re: They don't rely on drug tests for past use
*Nothing* on the SECRET level or above is ever deleted.
Back when I was a GS overseas working on things *nothing* was ever deleted.
Accounts and mailboxes would be disabled but never deleted.
They were just moved into an inactive bin/database/sub-OU.
Depending on the work you were doing, sometimes the entire laptop or desktop drive was imaged and stored as well as a "just in case" measure, but I only ever saw that happen with retirees because once you get to that level of fun you're pretty much a lifer.
I got out of the military, they inactivated my accounts and clearance.
I went back in as a GS 6 months later and they re-activated it all and it was all still there.
Got out of the GS job and went back in couple of years later as a contractor and when they re-activated my clearance and re-issued me my accounts all my old email and files were still there.
*EVERYTHING* you do on a secure PC or other device is never deleted.