A marker for use with hidden units! New Bolt Action rules for hidden units in the Bucharest book, or if you feel for it we use the following house rules.
“All units outside of Line of sight (LOS) are hidden from the opponent, and marked by a token. The Token represents units that the enemy havent sighted. They are hidden until they enter LOS from a enemy or are X inches from enemy unit OR fire their guns and revealing the position. When revealed the token is replaced by the unit.”