I’m a graveyard fanatic so I’ve been around these things quite alot.
But, then again, maybe my memory fails me (I haven’t actually been able to hunt graveyards in a long time, and my graveyard group doesn’t talk about fun things like that, only legislation and getting people to respect graves). Maybe I’ll ask the other people in my group which it is - “in memoriam” or “in memory of”...Actually, I think it is the latter. Sorry if I messed this up.
Whichever phrase it is, you’d see it only sometimes, and for some of those, it’d be obvious the person was “missing”. Once I ran across quite a monument in a graveyard in CT (I hunted all over there) where it told the story of how the son and a friend went out to CA, got on a ship and were shipwrecked. They raised the monument in that burial ground “in memory of” (yes, I think that is the phrase), knowing they were lost at sea.
That reminds me of what Vice President John Nance Garner said, I think it was: “There were two brothers—one went to sea and the other one became Vice President, and nothing was ever heard of either of them again.”