As Kieth says, look inside some like/same brand aircraft configuration pages. Some of Carenado, Just Flight, Hanger Studios 713, LionHeart and others have been there. When looking through your library/hanger, click on a known aircraft, and then on "Configer" This will open the type/livery pages. The hidden aircraft is in the "Type" page. It will also have it's own set of pages there.
FWI when you do choose one of the hidden aircraft, it will replace the original place holder aircraft. To bring that one back, you have to choose it again. As authors are becoming aware of this, some of them are redoing their descriptions which fixes some of them.