Sorry to bump an old thread but I believe what socio said about Clonazepam being difficult to detect in most drug screens is probably right (and jake99's argument now seems to make some sense).
I was reading up on Clonazepam (on wikipedia) today and I came across this:
Clonazepam and 7-aminoclonazepam may be quantified in plasma, serum or whole blood in order to monitor compliance in those receiving the drug therapeutically, to confirm the diagnosis in potential poisoning victims or to assist in the forensic investigation in a case of fatal overdosage. Both the parent drug and 7-aminoclonazepam are unstable in biofluids, and therefore specimens should be preserved with sodium fluoride, stored at the lowest possible temperature and analyzed quickly to minimize losses.
Source - R. Baselt, Disposition of Toxic Drugs and Chemicals in Man, 8th edition, Biomedical Publications, Foster City, CA, 2008, pp. 335-337.
So I'm guessing that because it is unstable in biological fluids, it is hard to detect or only a small amount is detected.