Select "Buy on this lot" and you'll enter Buy Mode. Then hover your mouse pointer over the chest and one or more magenta lines will appear, indicating links to other objects. If it is linked to all 3 sarcophaguses it means you need all 3 mummies. If linked something else, that is what would trigger the treasure.
Thank you, that was very helpful!
It turns out that defeating the mummies is NOT required for the treasure to be in the box -- there were no magenta lines at all. I went back and checked each save for several saves, and the treasure was missing in all of them until I got to the saved game from my previous trip to China. I was exploring this tomb when my vacation time ran out and the game teleported me home, and something about that seems to have been what caused the treasure to disappear.
I restored from the old saved game and used "Buy on this lot" to move the chest from its usual place to the section of the tomb where my sim was, and the treasure was there. I had done the entire tomb in the later saved game, and I couldn't face going through the entire thing again, especially since this particular one is the biggest tomb in the entire World Adventures EP. So I've done the entire tomb, and I've gotten the treasure, just not in the same game.
But thanks to your advice, I'm not stuck anymore, and my sim can go on to have further adventures. Thank you!