Cal. FAM § 17502

A local child support agency that is collecting child support payments on behalf of a child and who is unable to deliver the payments to the obligee because the local child support agency is unable to locate the obligee shall make all reasonable efforts to locate the obligee for a period of six months. If the local child support agency is unable to locate the obligee within the six-month period, it shall return the undeliverable payments to the obligor, with written notice advising the obligor that (a) the return of the funds does not relieve the obligor of the support order, and (b) the obligor should consider placing the funds aside for purposes of child support in case the obligee appears and seeks collection of the undistributed amounts. No interest shall accrue on any past-due child support amount for which the obligor made payment to the local child support agency for six consecutive months, or on any amounts due thereafter until the obligee is located, provided that the local child support agency returned the funds to the obligor because the local child support agency was unable to locate the obligee and, when the obligee was located, the obligor made full payment for all past-due child support amounts.