Gets weirder and weirder, doesn't it?
Get too close a look at an Un-men vessel and it destroys itself. Misjump on purpose, dive into the sun, catastrophic re-entry, it doesn't matter. There's going to be nothing left to glean any clues from.
Are the Un-men capable of deciding when the self-destruct? Or is something watching them some how and passing along an order?
Of course, a system in which an Un-men vessel is destroyed
never ever has another Un-men vessel arrive for trade again. Whomever or whatever is dispatching those vessels seemingly crosses a system of the list when a vessel fails to return.
And at just what frequency do Un-men arrive to trade? It would be even more eerie if the Un-men arrived and traded in a flurry of appearances only to "disappear" for a generation or so. News out of a cluster of backwater systems that the Un-men have appeared again - coupled with the usual flood of excellent quality pharmaceuticals at relatively cheap prices - would send traders, megacorps, and governments scrambling, but the Un-men will have gone before anyone of any consequence can get their teams to the region.
Like I said, there are entire campaigns waiting within this horror...