He's drunk at the bar
One night a police officer was staking out a particularly rowdy bar for possible DUI violations. At closing time, he saw a fellow stumble out of the bar, trip on the curb and try his keys on five different cars before he found his. The man sat in the front seat fumbling around with his keys for several minutes.
Meanwhile, all the other patrons left the bar and drove off. Finally he started his engine and began to pull away. The police officer was waiting for him. As soon as he pulled onto the street, the officer stopped him, read him his rights and administered the breathalyzer test to determine his blood-alcohol content.
The results showed a reading of 0.0.
The puzzled officer demanded to know how that could be. The driver replied, "Tonight I'm the designated decoy."