Or possibly ... it was stolen off the front porch.
In this economy ... that seems to be happening more frequently ... at least around these parts. Thugs follow around the UPS, FedEx and USPS trucks just looking for an easy 'grab' ... when a package is left and no one is home.
... and it only takes a few seconds.
Next time ... if it is expensive ... get 'signature required'. The recipiet will have to go pick it up or make arrangements for soneone to be there the following day ... but at least they will get it.