False positive oral drug test causes?

Because my son stole vodka and is deemed to have a drug problem, I am now court ordered to random drug screening. My first test came back positive for cocaine. I do not use cocaine or any other illegal, prescription or over the counter drug. In my research I found that someone with kidney disease can test positive for cocaine and I have 6 of the 10 symptoms listed, however, the site implies that this false positive only occurs in urine tests and my test was an oral swab. Can anyone tell me WITH CERTAINTY that this can happen with an oral test as well? I already know the causes for false positives are diabetes, liver disease, kidney disease, kidney infection, amoxicillin (sp), and tonic water so please only comment if you can give me evidence that a false positive can occur in the oral test for whatever reason. Thank you!