When someone “can’t be found” after their spouse dies, the question to ask is “did they marry again?”
In locations where women change their last name upon marriage, the reason for the disappearance should be obvious: the name has changed. Men who remarry do not change their last name, but it is possible that they move to a new location after the marriage or relationship starts. The move may be unexpected if the individual has lived in the same place their entire life.
If the person marries near where they had always lived, finding the record and new names might not be a problem. In other situations finding them may be more difficult. Obituaries, estate records, and newspaper “gossip columns” can be some places to find what happened to this person who is “missing’ due to a marriage.