The Indiana Jones Serpent's Chest mission will see you finding three ancient keys hidden in the Vatican, Gizeh and Shanghai. A couple of these are quite well hidden and can be extremely hard to find thanks to an unhelpful map. You'll also need the rebreather you buy in Sukhothai to reach two of them, so there's a lot involved here.

If you want to find all three keys for the Serpent's Cheat in Indiana Jones then I've got a full walkthrough here to guide through all the locations you need to reach

You'll find the Serpent's Chest in Voss' camp in Indiana Jones. It's in a smaller side tent near the bigger main tent. Finding it will trigger the Serpent's Chest mystery and say that you need to revisit previous locations, specifically the Vatican, Gizeh and Shanghai, to find the three keys needed to open it.

If you haven't already got the rebreather, so you can breath underwater in Sukhothai, get it before you try to find the keys as you won't be able to reach two of them without it. Once you've bought that in Indiana Jones and the Great Circle you can backtrack and find all three keys in the locations I've outlined below.