I was going to suggest there's another possible scenario. You have the car in the ditch or the accident or whatever, and the person is okay; he's out of his vehicle. The police arrive half an hour later, and he's drinking a beer. This may be because he's trying to avoid any suggestion that...he's going to screw up the evidence, but he's drinking a beer.
So now you have the alcohol, and within the last three hours he's been driving. The screening device would then provide.... If he fails it really badly, you take it to the proper test, the approved instrument test. The guy's now blowing 0.18; he's in the reverse two-beer defence. He's taken one beer, it'll never have gotten him to 0.18, so you could extrapolate. The police officer arriving at the scene would not at that moment have been able to make the immediate connection between the two. He might be able to do it a little bit later.
I admit, though, that we were thinking the hospital scenario and we're making these up as we go along.