Unoccupied locations used for delivery of illegal goods. It's actually considered terrible opsec in the darkweb drugs community. Making calls and doing paperwork to get things delivered to an invalid address is suspicious. Having lots of traffic where you get your mail and don't sleep is suspicious. And warrants will come cheap and easy if it can be demonstrated that nobody actually lives there because legitimate residences have more legal protections. A good darkweb vendor will refuse to ship to an obvious dead drop. But bad opsec is unfortunately not uncommon.