Legal Question in Landlord & Tenant Law in California

Turning it the keys gets you out of an unlawful detainer lawsuit, but it doesn't relieve you of your obligation to pay rent, plus any damages to the property in excess of any deposit, plus the landlord's rent losses from when you moved out until when they get another tenant in the property paying rent. So you have three choices, settle up directly with the landlord if the landlord hasn't sold the debt to the collection agency - just using them to collect for a fee, settle up with the collection agency - but be sure to demand confirmation that they legally have the right to collect the debt and that it will be fully discharged once you pay them, or ignore it and wait and see if they sue you for the money.