Devil is in the details. I don’t know if the agreement requires NASCAR to double verify using the extremely pricey but gold standard test of gas chromatography if there is a question about presence of a drug. Chemical tests can and do give misleading results.
Most drug programs use screening methods, but generally when there is a hit, they will do more sophistigated testing prior to handing out sanctions.
You’re right, there are many OTC drugs that can result in a positive drug test. For instance, if you have a cold, and take Sudafed before a drug test, you could test positive for amphetamines. That’s why it’s important, if you have to take a random test to inform the tester of anything (supplements or OTC stuff) you’ve taken in the previous week.