To access all enemies and "official" Secrets in PC Plutonia
level 7, Caughtyard:

(Note: For a YouTube video covering this topic click here.)

As you approach the level's exit portal at the northeast
corner of the map, you'll cross a floor trigger/"tripwire"
that will open a pair of compartments far behind you.
These 2 compartments are linked together virtually to form
a single score-affecting Secret.  Stepping into either one,
or both, will give a 100% Secrets-found score.

During a normal game these compartments contain enemies as
well.  If you've been getting a Kills score below 100%, it
might be because these enemies were overlooked.

Click here for a map of the score-affecting Secrets areas.