Missing Items on Doom 64 level 15, Dark Entries:

On this map, "official" Items include 2 Soul Spheres and
the Berserk Pack in the chamber across from the red key.

What remains on the map includes a backpack hidden behind a
wall panel in the blue-key room, green or blue armor vests,
normal health packs, and weapons and ammo, none of which
count towards a player's Items score.

A low Items score is normal for this level, and is due in
part to a flaw in map design: The armor helmet bonuses that
appear in the blue-key room (whenever you choose the wrong
switch) are not correctly tallied when calculating the
Items score.  (Armor helmet bonuses normally count towards
an Items score, even though armor vests do not.)