its nothing, probably a piece of the level that gamesas decided to leave out. like the ceiling above the hellion that was originally supposed to fall on it or whatever it was intended for.
Heres a definite problem, anyone found the mercenary level 4 secret yet? i cant find it at all. it says how to do it in your D3-FAQ in your c/games/descent3/language/readmes
HERE IT IS
SKIP THE FIRST SECTION AND READ 4.14 FIRST
[4.13] What about the Manual Pilot Powerup for the Guidebot?
 ------------------------------------------------------------
 Actually this is already to be found in the original D3 Retribution 
 HOG but it has not been used in any of these levels. As well, the 
 occurance in Mercenary level 4 seems to be a bug since the use of 
 it does not have any effect. Presumably the scripting routines just 
 weren't finished in time and someone accidently placed this powerup 
 instead of a working one in Mercenary level 4.
 Anyway, here's how to get it: there's one room with four large 
 pillars that connects three levels of the planetoid. Fly up to the 
 top level and then go straight ahead through the middle tunnel and  
 over a small pond. Pass an insectoid generator to your left and shoot
 the second 'mirror forcefield' on the left side. In the room behind 
 you'll find the 'new' GB powerup (which has some striking similarity 
 to a joystick 
) as well as an invaluable energy to shield  
 converter.
[4.14] I heard you could meet Jeff Slutter & Nate Goudie in Mercenary
 ------ level 4?
 
 Well yes, but they are hard to find! Exit the honeycomb mentioned 
 above and take a close look at the texures on the opposite wall. 
 There's a flythrough texture you should examine to give both of them 
 a yell 
			
Give us a D4 beta