To the extent we know unicorns don't exist. And it's the Neath, perhaps unicorns do exist, but the cards we have documented are pretty much the only places it'd make sense to put cards (i.e. much-travelled areas where the opportunity deck is relevant for a longer while).

The Adulterine Castle has been checked. Irem, too, will be. Perhaps you could try Hurling or Advertising, or whatever else, but you'll likely waste time unless you find a unicorn.