I molded chocolates today (using 2 different types of molds) and had the same problem -- a super fine separation between the top (what I call the cap) and bottom layers of chocolate.
To mold the chocolates, I coated the mold once, let it rest in a cold room, coated the mold again, let it rest agan, filled the chocolates (one a ganache, the other a mousse), and then closed the mold with a layer of chocolate. Cutting into a chocolate, the thickness was consistent around the filling. Also, I use a professional melting and tempering machine. There were no other problems with the chocolates.
This is the first time I've seen this issue. Any ideas? Thanks so much!