If one of the bombs is found and it is reported to the GEDEX explosives expert unit, the officers will detonate the bomb in a controlled, safe area, ThinkSpain reported.
But many people who find mortar grenades, projectiles or aviation bombs take them home.
Officers warned residents that there is still a chance the bomb can explode.
The bomb's security devices have failed over the past 70 or 80 years, the officers said.
Recently, a resident in the town of San Martin de la Vega suffered serious injuries after he tampered with an artillery projectile, ThinkSpain reported.
Those that find bombs should call emergency services, mark its location and not go near the explosive, GEDEX members said.