What happens when a car repossession agent can not find car?

They continue searching for it. Repo agents investigate. They don't just give up if the car isn't waiting for them when they show up. They'll figure out the habits of the debtor, they'll keep a close eye on them... basically, they'll make it pointless for the debtor to hold onto the car, because the risk of them hooking and booking the vehicle will always be there anytime the vehicle is taken into public.

That's on the recovery agency's end.

As for what the lienholder will do, that depends on a number of things. The lienholder is the lawful owner of the vehicle, and thus, has legal options available to them.