APE STORY LINGERS CAM RANH BAY, Viet-nam - With binoculars and sonar, people probe Loch Ness for its monster, expeditions search the Himalayas for the abominable snowman, and the guards at the depot here keep a watch for Powell's Ape. It has been nearly three years since the monster was almost seen by a bleary-eyed guard. Detecting movement on the perimeter, the guard fired and the beast fled. When morning came the area was examined and a footprint and trail of blood were found. The footprint was neither human nor ape. A picture was taken of the track but no one could decide who or what had made it. It wasn't long before Capt. Powell, then a depot company commander, found his name associated with the monster. All that remains now is a small photograph on the ammunition area's orderly room wall. Perhaps the legend of Powell's Ape is all there ever was, but the guards at the ammunition area here still have something to think about besides Charlie. ('Army Reporter,' 27 Apr. 1970.) - From the records of Bernard Heuvelmans -