When I was younger I actually (sadly and I know it was very very wrong of me but I was in a very bad and different place) had experience with something like this. The first time I hadn't actually drank that night, I had slept about 5 hours and it was almost shut-off time for the stores to sell alcohol, so I drove up there thinking I had slept and it had been hours so I was fine to drive. The problem was I was very underweight and never ate or drank anything but alcohol and alcohol tended to be much more potent in my blood stream than it would be for a normal person (I was once ordered to the ER by the police after they were called b/c someone thought I wasn't breathing, I hadn't drank in 5-6 hours, I'd been alseep and I'd only drank 2 glasses of wine, but my bac was .30). So anyway my car broke down when I got there and I walked to my brothers, we got in a fight and my mom called the police, not physical but she thought it could get that way. Well I freely admitted I had drank earlier that night but not since I'd woken up, they did the eye test and were certain I was under the influence and would be over the limit but they didn't breathalyze me and weren't concerned with charging me with DUI, they just had one of the officers go see if my car was where I said it was and wasn't crashed somewhere and let my mom take me home.
Another time I had been drinking that night, not a lot, but again for me it's a lot more potent than for normal people and concentrates my blood a lot more, but I was scared if I stayed home I would just drink myself to death so I drove to my mom's, then got in a big fight with them and didn't want to stay there and wanted my keys back which of course they wouldn't give me, so I said to call the police b/c they were stealing my car lol O.o So they did and the officers did the eye test again and said I was def under the influence, and it was well-known I had been drinking and driven but again they weren't concerned with charging me with dui, just me staying there. I'm told it's because they can't prove exactly when you drank and there was alcohol in the houses I was at on both occasions and they couldn't have proven in court that I hadn't gone in and drank before they got there.
My brother had a friend that drove to his house and crashed her car drunk a little ways from his house; everyone there knew it and she was being belligerent so someone called the police. They said they weren't going to charge her with dui, leaving the scene of an accident etc maybe but that she could stay there that night or something, but she kept arguing with the cops so they took her in. It's very difficult to prove unless there's eye-witnesses that saw you drinking and then drive or unless the police witness you driving while under the influence, it can be done but it's pretty difficult with a good lawyer.