Every day we have vehicles that are stolen by using a key that the victim left in the car.
Suspects are very skillful at finding the hidden key.
Often a suspect who breaks into a vehicle to steal items will find the key or a firearm and the result is more crimes. Rarely do the have to spend much time looking. Lock your vehicle and engage your alarm system. Driving around town in a vehicle with a shattered window is a pretty good indicator that the vehicle is stolen. Alarms do deter some thieves.