Near the 4-th Trident and the Levelgate to Costal Ruins. Crossing the Levelgate and coming back allows to re-get this secret (dliiiinnng... again), increasing the number of secrets in Statistics, an unusual counter-bug.
To open the trapdoor: explode all the bones on the ground, use the crossbow, laser viewer, regular arrows.
Since I'm playing "pistols-only-style" as much I can, I had to consult Theresa walktru to get it. Secrets #3 and #4 seem unaccessible without Secret #2. Allow me to say that's the more unfair thing in The Last Revelation... :-(
This secret is available up to the end of the level