Toxicology Tests False Positive

[KatKat10]

Has anyone undertaken a toxicology test for substance misuse and that test has come back with evidence of use but you know you have not taken that substance recently? Has anyone had a test result come back showing use of a substance, even though the test might show the historical & admitted use of that substance, ingested due to slow growth of hair? Are there any instances of environmental factors that could produce a false positive in hair, food, hair products etc?