Is there any reason a urine tox (taken @ 12pm today)could show negative for benzodiazepines after having taken a .5mg klonopin (clonazepam) @ approx 3pm yesterday?

Many reasons: Depends on several factors - what test are they using? Tests for benzodiazepines usually detect oxazepam, which is a primary metabolite of most of them (the ones where it isn't - xanax (alprazolam) and klonopin). There are test kits that will reliably detect Klonopin and xanax (alprazolam), but these are not always used. In addition, if you drink a lot of water, you may dilute your urine down to a negative level.