If your liver is sufficiently screwed up, and you have the rare enzyme disorder below, your body will take in carbs and ferment them into alcohol in your belly. And then, you get legitimately breathalyzer drunk.
The entry above says it’s rare, but then it says liver problems can cause the condition. So perhaps some alcoholics destroy their livers, end up with ABS, and then people assume they’re lying about not drinking.
Or, maybe some people some of us go around as if we’re half-drunk anyway. What if when we drink a small amount we get fully drunk? Maybe someday they’ll find out it’s common.