In a way, your point about authorities and highway patrol being alerted is actually a good thing. These trailers can go in lockdown and wait for authorities to arrive. Even if a human were on board, a silent alarm could be triggered without the human making any obvious moves for distress (perhaps a hidden panic button in case sensors don't trigger, assuming a human driver is required to be with the trailer). Also, trailers today aren't immune from hijacking/piracy if the driver isn't armed.