This had been perplexing me for a while and I think I figured it out. When the player crawls out the pipe and arrives at the epic cliffside panorama in Surface Tension, then proceeds to drop down the ledges to get the lower levels, there is a red laser beam which if tripped, is supposed to activate a turret which then attacks the player — I think, in fact, theoretically, when the nearby soldier is alerted to your presence, the turret can activate too, or perhaps the soldier himself trips the laser. Anyway… I remember maybe only one instance when I played through this level that this actually occurred. All other times, the turret seemed to be mysteriously missing.
What is actually happening I believe is when the map first loads, the turret is spawning just above the ground such that when it lands due to “gravity”, the physics engine is causing the turret to tip over and fall down to the bottom of the cliff…so it’s not actually missing…it’s just fallen a couple thousand feet or so out of range 
When this map loads, if you immediately rush out of the pipe and turn right (I suggest you noclip so you can float in the air) , you can actually catch a glimpse of the turret falling to the ground in this manner. I’m assuming this can be fixed if the turret can be spawned at more safe/stable coordinates on the map, or simply not so far above the ground so that it won’t tip over when it lands immediately after spawning.
Hope this helps.