Check the former tenant’s rental application for personal identifiable information such as social security numbers, previous addresses, or emergency contacts to narrow down their location.  Send a letter to their last known address with “Address Service Requested” on the envelope; if they have filed a change of address with the post office, you will receive a notification with their new forwarding address.  Utilize online people search engines or public voter registration records to locate their current residence if the postal method fails. Hire a private investigator.