Legal Question in Landlord & Tenant Law in California

You don't say if they moved out at the end of a lease, at the end of a month on a month-to-month, or what. Did they just slip away in the night? The circumstances of how they left affect what you can and have to do.

Point out to the tenants that legally until they return the keys they are still considered to be occupying the rental unit and clearly liable for rent for that period of time. Once you have made sure that they do not intent to move back in, you will need to change the entrance door keys/locks.