Well no your not alone. Lots of people have had this problem. The thing is; it is not a regular tombstone unless you kill all your crewmates [ then you get tombstones and game over. ]
Also I do not see that you have not stated how many crewmates you have. But if you go to each area and look for a rock mound with a Daisy sprouting out of it as you clear the area. Click on it to pick the Daisy and then make the Death Daisy potion on the Hardwood crafting bench.
Once you find it. You can mark the spot with a garden gnome if it is in the dense area where there is foliage hidding the stone mound. This will make it easier to find again so you can use the potion to bring them back to life.
Just remember to look on the ground and clear the area. It is a grey colored stone mound with a flower sprount on it that your Sim can interact with [ like it will say mourn and that you have to do before you can pick the flower. ] and it can be found most anywheres on the island where you last saw them.