Inaccessible secret chamber on an exterior wall in Master
Levels map Bloodsea Keep (PS3/PSN version):

When you first appear on this level, you'll be at the south
end of the map, and you'll be facing northeast.

This map is dominated by a single large, main structure.
Along the outside west wall, near the structure's northwest
corner, is a 5-sided protrusion.  This is the western half
of a large chamber, which contains one of the 4 "official"
(score-affecting) Secret areas on this map.

In the original PC version of Bloodsea Keep, access to that
chamber in Single-player and Cooperative games depends on
the death of a nearby Mancubus: When that monster dies the
walls surrounding that chamber drop down, allowing access.
(Note that this monster only appears at Skill levels 3 and
up, meaning that the chamber remains closed in Single and
Co-op games played at lower Skills.)

In the PS3/PSN version the chamber cannot be opened at any
time in Single-player or Co-op games, due to an error that
was introduced when porting the original PC map.

In all versions of Bloodsea Keep, that area can be accessed
during Deathmatch games (at any Skill level) since a player
spawn point exists within the chamber, as well as a switch
that will open the exterior wall segments.

Click here to watch a YouTube video showing how the chamber
opens in a Single-player game on PC, but remains shut under
similar circumstances in the PS3 game.