This is actually a very plausible idea. Someone found the pyramid before the Germans ever went in, explored the whole thing, learnt all of its secrets and left with a few special items he knew would be needed later. At the same time he did something that "awoke" the W'rkncacnter (at least subconsciously) so that the monsters began to appear.
This would bring about another question though, what else did this man take out with him? Why would he leave all of the different pieces of treasure and only carry out alien pipes and a gold key? It is possible that there was MORE treasure and this guy carried it out. Either for his own greed or to prove he had found some weird pyramid.
Unfortunately there is no way to confirm if this is indeed what happened. Someone found pyramid, figured it out, awoke it and left. From there we know the story. Germans somehow obtained the info and stuff and from there decided to go to the Yucatan.
Original post:
: Here's my idea for the "knowing of artifacts, but not of aliens"
: bit: One of the dead bodies tells us that the pyramid seemed to awaken
: behind his team, trapping them. Perhaps someone scouted the pyramid long
: ago, and their reports were used by Muller. Perhaps there were no monsters
: then, and this first party's incursion awakened the W'rkncacnter for the
: first time. When Muller and company arrived, the monsters were either
: still active or merely dormant.
: This would mean that the monsters Sgt. Eddings sees and fight from the ground
: floor and down are there only because the rest of the Special Forces team
: woke them up. If Eddings had gone in first, he probably wouldn't have seen
: anything at all, for a few floors at least.