How can you have units with secret locations on the board?
There are a few popular ways.


Have a duplicate game board with a referee to validate things.
Have a map of the game board set aside with the secret locations marked.
Have numbered locations on the board where the hidden units can be and mark the number down secretly.
Use decoy markers.  Place several markers on the board and treat them like a real unit.  Only one of the decoy markers represents the actual unit.