I'm sorry to hear what has happened to Mr. Andrew's cars and property.
Glad and thankful that he's in good health, one of the true irreplaceable goods.
Given the relative rarity of these autos, one would think that the specialist collector-car market was an exclusive and familiar circle. Public sales are celebrated with automotive news coverage, and details of private sales exchanged in whispers by smaller crowds of people around the biz. Unsure how much remains truly hidden.
Did not know that there existed shadows in which ultra-rare cars were coveted to the point of illicit acquisition. One would suspect that this specific and specialized heist might already have its specific and specialized buyers used to this sort of shady channel, so I am unsure if the following suggestion may help:
Besides the relevant authorities, perhaps a planned social-media campaign increasing awareness may make these sales more difficult? A story and heist of this scale like this could (and would) get coverage by collector & specialist channels (SCM),digital & print automotive news outlets (C&D, Jalopnik, Drive), and perhaps even a bit of national coverage?
Perhaps a blanket of general awareness may mark a red-flag on these 28 cars.
A succint youtube video may be easily and quickly disseminated. A condensed two to three minute version of the interview along with a few infographics would do. It could be part of a 'hub' site listing as much info on the case: the cars, investigation progress, coverage, and tips.
Best of luck!