I'm not really sure about these breakdown products. If I look them both up with Wikipedia, the nitrogen-bridges in cocaine as well as as its metabolite stick out like sore thumbs. I don't think there are many natural sources for those compounds, especially in the thin soup of pee and poop of a sewer system.
The milage may of course vary for other drugs. MDMA (ecstacy) has a rather odd ring in its structure which is not common as well; morphine/heroin/... have a really weird N-bridge spanning across a nearly flat molecule. I therefore suspect that these compounds and metabolites can be easily traced. This of course is not the case for amphetamine (speed) or gamma-hydroxybutyrate (GHB, 'the date rape drug'), and to a lesser extent THC. These chemicals are 'simpler': amino-acid like, a few longer straight carbon chains, and so forth. Our bodies should have an easier time with those, and thus break them down further.