TamlynJones, you are very insightful. I think both categories that you mention are significant factors.
First off, I think it safe to say that a large amount of casualties occurring in a limited area in a short period of time correlates to a large percentage of MIA, since burial parties cannot afford to take the time to research each body. The dead were often soon visited by scavenger soldiers that hung around the margins of both armies, emptying pockets and removing anything of value from the corpses. Letters and diaries that could have identified the bearer were often discarded as worthless, and blew away with the wind, or dissolved in the rain. Of course, inscribed watches and rings would become spoils of war.
The other broad category, as you mention, are those soldiers who died alone and in remote locations. Their gravesites were therefore more readily overlooked and forgotten. A few years ago, a soldier's remains emerged from the soil near the famed railroad cut, where millions of visitors have passed. I am also reminded of fragments of a hand that emerged from a well at Herr Tavern, which was dredged because the water began to taste and smell bad after the battle.
There is a third category as well, which typically originated in the field hospitals, but extended all over the field. Many graves that were well marked at the time on wooden headboards were not well tended. With the passing weeks or months, the headboards might become dislodged and moved some distance from the grave, or else the elements eroded the names that might have been initially written in pencil. By the time the dead were removed in the early 1870s, many had become nameless.
Of course, less attention would naturally be devoted to the enemy, in this case the Confederates, so their MIA rates would be expected to readily surpass their Federal counterparts. Even so, there are still many unknown Federals in the National Cemetery.
Bringing the