Many times, with packages, thieves will follow postal workers (or UPS drivers) around and when they leave packages on doorstops- the thieves will come after them and steal the packages. Happens more than you know. If you're ever worried about it, always ask the sender to get a signature. More of a pain for you to get it, but at least you know you'll get it. My DH works for the post office as a carrier and more than once he's caught people stealing packages off of porches (and not always in bad neighorhoods- the thieves know rich people order good stuff). There's also been times when a supervisor has asked if he left a package at so-and-so house and he says yes, but the package wasn't there when the people returned home. He wouldn't risk his job and prosecution on a package- but an opportunist thief would have no problem with it.

Sucks, tho.