Call me cynical, but I can easily imagine the scenario where the authorities themselves drop off a suspicious package as pretext to do a search, so they do not need to bother with that pesky problem of obtaining a warrant. Numerous agencies have been documented using this sort of dishonest and dubious tactics, so I do not think that possibility could be dismissed at the start.
At this point, I would trust some random unattended package more than I would trust the government to act with integrity. And that just makes me hang my head in deep sadness and shame for this country.