I was reading through some of the things that the dead men say in PiD for my Muller fanfic and I came across this one.
"Yes, this has happened before. This same weird, speechless,
disembodied sort of conversation. I don!t remember more, sorry..."
It seems that this was said by the anonymous german soldier found on the Ground Floor of the pyramid. He says that he recognizes the fact that he's had a conversation like this before. That means that someone came around after his death, but before you arrived and used the yellow crystal. This makes one wonder, how did the previous speaker get a hold of the yellow crystal? And why was he there?