I once failed a screening test while driving a car - despite not having drunk a thing for weeks prior to the incident. Although I have no proof, I suspect that my mouthwash had caused me to set the machine off, as I had used some about an hour before. Fortunately there is a second screening test here in NZ which doesn't just detect the presence of alcohol, it measures the breath alcohol content. After passing this I was able to proceed on my merry way.
Surely something similar could be used in situations like this to save all of that wasted time, money and tarnished reputations...