My son did that too - he unbuckled completely and would stand up in the car. This behavior started on a long driving trip last summer when he was 2. We were out of town for a couple of weeks so didn't have time to think about the police officer strategy (brilliant idea!) so I'll tell you what we did do, although I don't think it was terribly safe, but we were at wits' end and it wasn't like we could just stay where we were or keep rebuckling him every 20 minutes on an 8 hour drive. And nothing we said or any consequences we could think of made any difference. Since we had our dog and luggage in the folded-down back, there was no space for a parent to sit back with the kids in the second row.
We used plastic ties (the kind like you use on a lint filter - they go through the other end and fasten) to prevent him from unbuckling. I made sure there were scissors in the glove box in case of emergency, but I didn't like the idea of having him trapped in case of an accident. (His father's strong enough to break the ties, though.) But it was better than having him standing up in the back of the car at 70 MPH! The good thing was that he hated being restrained so much that he quickly agreed to stay buckled on his own.