My wife's great-grandfather William Frame Apgar was born William Frame in Chicago around 1888. Around 1918, he disappears, estranged from his wife---my wife's great-grandmother. Perhaps he enlisted in the war, perhaps not. None of my wife's immediate family knows what happened to him.
It is possible that his siblings might have known what happened to him and passed that information down. My answer to where William Frame Apgar went might rest in the descendants of his siblings.