I'm afraid they spawn randomly.
When you say you've explored all tombs, do you mean all tombs in all countries? Or just all in China? Because they appear in all 3 countries. If you're not sure whether you've explored absolutely all tombs, check the list of tombs (or tomb trophies) on the country guides on carl's guide.
Also, I think that they might respawn if you go through tombs again. The traps will be fixed because you did it before, but I think treasure chests and coins will respawn. Does anyone know if I'm right about that?