Player gets a score of "SECRET 0%" on these Doom II levels:

LEVEL 14: THE INMOST DENS
LEVEL 21: NIRVANA
LEVEL 25: BLOODFALLS
LEVEL 29: THE LIVING END
LEVEL 30: ICON OF SIN
LEVEL 31: WOLFENSTEIN
LEVEL 32: GROSSE

Some of these levels do have hidden or secret areas, but
not all versions of Doom II score these areas the same way.
In some editions of the game prior to v1.9, "SECRET 0%" was
the normal result at the end of all of these levels.
In Doom II v1.9, it is possible to get SECRET scores above
zero for some of these levels.

o Doom II "Secret" scoring, comparison of v1.666 and v1.9:

                        | VERSION 1.666   | VERSION 1.9
                        |-----------------|----------------
  MAP | MAP NAME        | SECRETS : SCORE | SECRETS : SCORE
  ----|-----------------|---------:-------|---------:------
  .14.|.THE INMOST DENS.|....0....:..0%...|....0....:...0%.
  .21.|.NIRVANA.........|....0....:..0%...|....0....:...0%.
  .25.|.BLOODFALLS......|....0....:..0%...|....2....:.100%.
  .29.|.THE LIVING END..|....0....:..0%...|....0....:...0%.
  .30.|.ICON OF SIN.....|....0....:..0%...|....0....:...0%.
  .31.|.WOLFENSTEIN.....|....0....:..0%...|....4....:.100%.
  .32.|.GROSSE..........|....0....:..0%...|....6....:.100%.
  ---------------------------------------------------------
  Notes: "Secrets" are the areas that are considered secret
  by the game.  These areas count towards a player's SECRET
  score, shown in the Tally Screen at the end of the level.
  "Score" is the normal maximum SECRET score attainable in
  this version of Doom II.
  Some of these maps have additional hidden areas which are
  not considered "official" Secrets by any Doom II version.
  Note that the "maximum scores" above apply only to the
  game engines released commercially by id Software.  Some
  third-party Doom engines will automatically give scores
  of "SECRET 100%" for all levels which have no "official"
  Secrets.