Sounds fishy to me too. While it (porch theft) is certainly possible and does happen, it is fairly rare. For example, I've sold stuff on the side on both Ebay and Amazon for at least a decade. In that time I've must have had at least a few hundred packages left by the door/front porch. Never once had a theft incident.
Unless the box/package somehow communicated that it was a high value item, I'd be even less inclined to think it was a theft scenario. Luckiest thief in the world? Possible I suppose. But if I were a thief, two things would motivate me to steal. A) easy opportunity and B) good likelihood that the item has high value. A nondescript small box with a few cables doesn't sound like "B".
As far as motivation "A", I would take a look at his residence from Google Earth. Some residences are clearly detailed. Scarily so. You can get a real good idea just how opportune a theft scenario might have been just by checking it out that way. A short driveway with good surrounding cover and fewer homes surrounding the property (with fewer potential eyes to notice a stranger) would probably be the best situation for a thief.