How are drug metabolism and detection related?
Short answer: Yes. 

Diazepine is a ring-structure that forms the basis for the class of drugs called benzodiazepines. This class includes long acting and short acting agents. If you're actually talking about diazepam, it's one of the long acting agents, which means the half-life of the metabolite in your body (and it's detection) is long.

The National Highway Traffic Safety Administration has good reasons to want to collect samples of various drugs non-invasively, and, indeed, they're headed toward saliva for screening. A neat little item called a Drugwipe can pick up traces of diazepam from not only saliva and sweat, but from surfaces people have touched.*

The real kicker, though, is hair, from any part of your body. Drug use(including diazepam) can be  detected in hair for months after (even single) use, and many employers are now going for hair sample testing to screen for drug use.

*Saliva and Sweat Testing With Drugwipe®.