I'm not sure if this riddle has been posted here before, but this is an old (not too great) riddle I've heard a long time ago:
One hot summer weekend there was an emergency call to the police that a crime has been committed, Someone has been murdered at the park! The only lead the police have from their live footage feed was that the perpetrator was covered in blood and what car they used to drive away in. They now know the car model and colour, but nothing more about the murderer or car! The police begin searching around the city for this car and one officer finds a house with a car on the drive way that matches the same model and colour.
The officer goes to the house and asks to speak to the car owner about the event. After questioning the car owner, the owner said they were at home all day and that no one has used their car all weekend. The officer asks the car owner if they could search the car to confirm their story, the owner gladly agrees and hands over the keys. The officer begins searching, looking for possible weapons or any traces of blood (since the perpetrator was covered in it). The officer finds nothing related to the crime, and not even anything incriminating (i.e drugs, weapons). The car was spotless. However after finishing the search, and some thinking, the car owner is arrested!
At the police station the owner pleads guilty to their crime of the murder.
How did the police officer know the owner was guilty?