Consider using a trained tracking dog. Contact local obedience class teachers and inquire about hiring someone with a trained tracker dog that is trained to locate humans and animals by scent. Don't give up. Keep looking in those same old spots, calling and listening. Don't be embarrassed and try not to let yourself become paralysed with grief and anxiety.
Cats are tough customers and can last many days without food or water.
They also can hide very well, and may not be rescued by animal control or an individual for many weeks after their initial disappearance. Don't give up the search too soon. Don't give up as soon as the cat fails to return home or after only a few days and don't just wait for the cat to come back or not. Keep looking and keep checking.