Drug tests are not specific to metabolites only from scheduled sources. Because unscheduled cannabidiols trigger drug tests the same way as scheduled ones, they are ineffective at identifying illegal use.

And yes, this gets challenged frequently since there's a legal concentration of ∆9 for products sold in the US naturally derived from hemp.

Generally, if you weren't caught in possession of something scheduled, not admitting to use when presented with a "positive" drug test is enough to have it dismissed.

I worked with two different people that tried that. First guy crashed a forklift and was told he couldn't operate heavy machinery while taking any kind of depressant medication, and the second one was an armed security guard who tried explaining that his paranoid behavior was from switching cannabis strains. Under federal law it's a felony to own or possess a firearm while a user of cannabis, and the court cases so far have not been friendly to medical users.