Secret

Secrets in the Doom series can mean several things:


 * Areas of a level that are hidden (to some extent). Usually, there is some clue about a secret area, such as an off-color wall that is actually a door.  By convention, most levels are designed so that it is not necessary to find any of the secret areas to complete the level.  After exiting a level, the percentage of the secret areas found is displayed.


 * A special sector type (9), used to calculate the secrets-found percentage displayed at the end of each level. The number of type 9 sectors in the level is the total number of secrets.  When a player first enters a type 9 sector, the number of secrets found is incremented by one.


 * A linedef flag bit (5). This makes a two-sided linedef appear on the automap in red instead of yellow, that is, just like a single-sided linedef.  This is used to hide entrances to secret areas, but has nothing to do with the calculation of the secrets percentage displayed at the end of each level.