Thanks for the advice, everyone. I'll be calling the post office today.
I highly doubt either of my neighbors who live directly next to me would have taken it. We all own our places. I've met both households who's front doors are next to mine, and they are normal, hard working, straight laced people.
@inter_ceptor00: I don't definitively know where this package was left, but other packages have been left next to my front door. It is outside, ungated, but covered by an overhang. Only two of my neighbors front doors are on either side of mine. Front doors to other units in the complex are located elsewhere. Anyone off the street could walk up a few stairs and take a package. Does this sound like it qualifies as an unsafe location to leave a package?