This thread is entertaining, so sorry to interrupt the fun, but it is most likely a wood chip or something similar that ended up glued in between the layers and got pressed into the board.
Had a few warranty claims from people over the years and that is the reply that has come back, no structural issues or problems with the boards in question either.
Shouldn't cause any issues with the board, but it would be interesting to cut it apart when you are done with it and see what is in there.