My wife has an ancestor who "disappeared" ca. 1918 shortly after he and his wife had marital problems. It is possible he changed his name. It is also possible he enlisted in the service during World War I and never returned to the area where he was from (either because he died or he simply chose to live somewhere else).
Another ancestor of my wife's died in his early 30s in the early 1860s. It is too early for a death certificate, but I do have his estate records. Now I am wondering if he died in the Civil War.
Keep in mind the time frame. Was there some national event that could explain your ancestor's absence or demise?