Hot Mustang

There was a bank robbery yesterday morning in Springfield. Several on-lookers
saw a red Mustang speeding away from the scene of the crime, but nobody got
the plate number, even though the driver had a near collision with a tanker truck
and had to slam on the brakes on several occasions to avoid injuring innocent
pedestrians. The Springfield police notified the authorities in the surrounding
towns, hoping to catch a lucky break.

The next morning, the police in a neighboring town thought they had a likely
suspect. They remembered that this suspect, who we’ll call Bill, had a late-model
Mustang that could easily have been the getaway car. And, because Bill had had
a few scrapes with the law, they decided to pay him a visit. So the police show up
with a search warrant, and explain the reason for their visit.

Bill was shocked and angered, and denied any involvement whatsoever. “Look,”
he said, “I’ve been sick for about a week, and neither the car nor I has left the
premises!”

“Show us the car,” they demanded.

Bill led them to the garage where he kept it, and there it sat, a beautiful,
practically new red Mustang, perfect paint job, 18-inch alloy wheels, leopard-skin
seat covers, the whole bit.

The two police officers trained their flashlights on the car, as each one of
them kind of walked around it, looking for evidence. Finally, one of the officers
said, “This car’s clean. It wasn’t involved in any robbery. I know it’s been sitting
here for days.”

What did the officer see that clears Bill and his car?