When Bobby went to the park with his family, his father accidentally dropped the keys to the car. For more than an hour, the whole family was searching for them. Bobby's mother instructed them to "leave no stone unturned, we need those keys to get home!" So the family searched in all possible places until Bobby's sister found them near the car. After that, the family could relax and enjoy the park.
The police is the city left no stone unturned in the search for the missing bank robbers. All leads were investigated and every contact was questions. The police had almost exhausted all of their resources when the thieves were found in Mexico living in luxury.
The expression to leave no stone unturned means to use all your power, energy, and resources to find something/one. In example 1, Bobby's family had to literally look under and around every stone in the park until the missing keys were located. In example 2, the police searched everywhere and followed all leads until the thieves were located.