Why does a blood test show ethanol when no alcohol was consumed?
There are some situations when ethanol-like compounds can be present in the blood stream:


  [...] exposure to hand sanitizers, after shave lotions, perfumes, medications, hygiene products, cosmetics, foods [...]  Uncontrolled diabetes mellitus can cause false positive alcohol tests. 


Source: Dr. Kokil Mathur. Answer on: False-Positives for blood alcohol content. Available from MedHelp forums.

Other causes as pointed on eHow.com:


low calorie diets
gasoline, paints, lacquers
lower hematocrit values
diabetes mellitus