EDIT - SOLVED:

Well, shoot. Nevermind. I just tried jumping around like an idiot on the ledge for a couple minutes - apparently, the trigger-space is somewhat small, and I managed to hit it by sheer luck.

ORIGINAL ISSUE BELOW:

While playing through the Killing Fields (level 2 in the Code of Honor campaign), I've only been able to get 4/5 secrets. I've looked at various guides, and they all pointed me to a ledge above the arrow-trap hallway. I've made the jump numerous times, yet am never given the criteria for getting the secret. I've even tried using any Nuke I have so that I can pick up the one in the secret area, on the off-chance that is what triggers the secret.

Yet nothing works. I remain at 4/5 secrets unlocked for the level. Has anyone else experienced this? Is this particular secret bugged?

(Side note: I've played Enter the Wang and Code of Honor through to completion - so far, I've picked up every single other secret in both campaigns. This is the only secret that seems to be bugged, as far as I can tell. I have not yet been through Wanton Destruction or Twin Dragon yet.)