A first cousin of my great-grandmother disappeared in the 1920s and was last seen in California and Colorado by various members of his family. he was approximately fifty years old at the time of his disappearance.
He was never found.
The last record he was mentioned in was the estate settlement of his brother. That brother died in the 1940s and his only heirs were his siblings and their children–including the missing brother. The judge overseeing the settlement of the brother’s estate declared the missing man dead in order to complete the settlement of the estate and disburse the balance to his children.
The estate settlement contained testimony from the missing man’s children regarding their father’s disappearance and what attempts were made to find him. The court record was the only documentation I could find regarding the man’s disappearance.
If your “missing” relative cannot be found by you and could not be found by his living family during his lifetime, was there an estate to which he would have been an heir? The settlement of that estate may mention what happened to the relative, confirm that he was missing, or provide information to assist you in your search.