Unless there is a trackable number on the package, it is nearly impossible to find it.
They handle millions of packages every day. So at least put Delivery Confirmation on it (the best service for the money), Insurance (which has its limits), Certify it, Express Mail, or Registered mail. The last three require a signature when delivered. Delivery Confirmation is scanned into the system at every stop and Registered mail is the most secure. It goes under lock and key with each employee, truck driver, etc. signing for it. If left overnight at a facility it is locked in a vault.