@mysteryman,
I suppose that such persons are wellknown to the special department at the German Red Cross as well.
The number of 1.3 millions reflects to those where they have no data at all - on tombstone it's just "missed in ....", if they are officially declared dead.
(My uncle was officially declared dead as well, and I've got all his personal belongings. That was in 1944, but it lasted until the late 60's when finally the location was found where he might have been buried.
When I was young, in nearly all families of my classmates there was at least one close relative, who was missed.
Because some still returned from Russia, the hope that others might return, was alive quite a long time. (And even today, some siblings, children don't/can't belive that their brother/father is dead.)