oh thank god my law teacher told me about this years back. I aint no lawyer and this is simply high school law course from like years abck so don't rely on me obviously. From what I remember, if you have the keys on you, that means there is an intention for driving under influence soon or later on.
Edmonton Alberta Legal Information on Impaired Driving and Licence Suspension
So whether you are sitting on passenger seat, driver's seat, or near by, if there's a reasonable suspicion that you may be able to control the vehicle, in motion or not, you can be charged for DUI as you still had the intention to operate the vehicle (or suspected intention). But, if the key is not in your possession, there's no way you can operate the vehicle therefore sleeping inside the car is okay as long as key cannot be found anywhere near you (passenger seat or your friend's pocket apparently doesn't count, because you can still start car and operate radio drunk from passenger side, law says in motion OR not therefore using radio also counts as DUI). From what I learned years ago, If you are inside the car but the key is stored on top of the wheel or inside the exhaust or something, they cannot prove your intention to operate the vehicle because it is not physically on you, and while you are inside an space, key is stored outside. Weird system, I never got it, but I avoid getting into car when I'm drunk period.
A friend of mine got charged for DUI few years back although he wasn't even in the car. Funny thing is, he didn't have keys on him so he could have disputed it. Unless that's the case, I dont think disputing this will work that well. Hope you can find a good lawyer.