Did you notice the tag next to the pieces? It says, "N13E2." What does that mean?
In the shelf at the end of the antiquities tent, you'll find a notebook with lots of similar notations in it.
Pick up the red notebook on the middle right shelf, and you'll find a map of the tomb with each section labeled. Can you find N13E2?
It's at the end of tunnel 2.
When you go to the tomb, pick up the shovel from the right table in the main room, then head into tunnel 2. At the end, zoom in on the pile of sand in the right corner and use the shovel to uncover the last piece.