The Canonical Defender, Prof. Ed Peters, has on his excellent blog In The Light Of The Law, an interesting post about marriage and presumption of death.
I suspect that this issue could include also military combatants who are MIA.
Thus, Prof. Peters:
Arthur Gerald Jones, declared dead in 1986 some seven years after he had simply disappeared under ambiguous circumstances, was recently found alive and well, working under the name of Joseph Richard Sandelli. Jones, whose religious affiliation was not identified, was married with children when he vanished, and it seems that he has not remarried since then, but, as technological advances make more likely the discovery of other erroneous declarations of death, the Jones-qua-Sandelli episode occasions three quick points about the canon law of “presumed death” per Canon 1707.
1. A civil certificate of presumed death is not sufficient basis for a bishop to declare a “survivor’s” freedom to marry in the face of a prior marriage per Canon 1085. An ecclesiastical determination of presumed death must be made.
2. In making that ecclesiastical determination, a bishop find evidence (albeit not proof) of actual death, and not merely of absence, however protracted.
For example, a man books a passage on ship, but he never shows up at the dock, and is not heard from again, even for many years. Such a fact pattern is insufficient for a canonical declaration of presumed death because there is no evidence that the man died. Alternatively, a man books a passage on ship and is seen on the ship before it sinks at sea with loss of life. He is not heard from after the shipwreck. That would be much closer to providing a basis for an ecclesiastical declaration of presumed death, because circumstances pointing to death are present.
3. If a spouse remarries after having received an ecclesiastical declaration of presumed death, but the original spouse is later found alive, the second marriage is null and a petition declaring such nullity should be filed, if necessary, by the promoter of justice per Canon 1674.
Read more about the canon law of “presumed death”: CLSA New Comm (2000) at 1798-1799 or GB & I Comm (1985) at 949.