Unfortunately sometimes this can happen, but 90% of the time we’ve found that the package was simply placed in an unexpected area at the delivery address. We first recommend asking your recipient to check any other locations the package might be (the front door, back door, mailbox or even a neighbor's house).
If the package is still nowhere to be found, it’s possible that it was erroneously marked as delivered by USPS®. If it’s simply a USPS® error, the package will usually be delivered the next business day.
Another unfortunate possibility when dealing with certain shared-delivery points such as apartment buildings or even porches in residential neighborhoods is that the package may have been stolen after delivery. In that case, unless you purchased a Signature Confirmation service, USPS® will not provide any recourse since they were only paid to deliver the package to the address, and not to ensure its protection after delivery.
If the package is still nowhere to be found, they can sometimes be located by the recipient visiting their local Post Office, who may be able to speak with the letter carrier for that address and provide more information. You can also try calling USPS’s main customer support line (1-800-ASK-USPS) or you can submit a missing mail search request.