Missing secrets on level 9, Nessus:

In PSX Final Doom, two maps have "Secret" areas which are
inaccessible to the player.  The player character cannot
physically step into these areas, and therefore the game
will not count those secrets as being discovered.

On map level 9: Nessus, there is a "secret area" in a
cubbyhole on the right side of a raised cul-de-sac, which
is in the the center of the north edge of the map.
You cannot physically step into the cubbyhole, and so the
game will not recognize that you have been there.
Depending on the skill level you're playing at, the
cubbyhole may either be empty, or may contain a BFG 9000.
If the BFG 9000 is there you can grab it (although it may
be difficult).
This cubbyhole is the only official Secret in Nessus.

100% Secrets can be attained on all other maps in PSX Final
Doom, with the exception of map 29, The Death Domain.