If you send someone certified mail and no one signs for the certified mail then the certified mail will eventually get returned to the sender.
The post office will usually attempt to deliver the certified mail for up to 7 to 14 days and then if no one signs for the certified mail after 14 days the USPS post office will then return the certified mail to the sender.
Usually after 7 days of no one signing for the certified letter the post office will put another reminder in the persons mail box to pickup the certified letter.
After that second attempt and usually when 14 days pass the certified mail will be returned to the sender but sometimes the USPS will send the certified letter back to the sender even sooner than 14 days.