Well, if your certain you haven't mistakenly taken a benzo-containing substance, it's possible that the Ambien is the culprit. Those "Z" drugs (Imovane = zopiclone; Ambien = zolpidem) are so closely related to benzos, it's possible they're detected as the same thing.
I'm winging it on that one, but molecularly speaking, they're very similar and equally addictive or dependence, depending on your definitions.