I would guess the story is either someone during the war decided to keep it to play partisan, or someone came into it at the end of the war and decided to keep it for a future rainy day, or, less likely, some German soldier hid it with the hope of returning for it. My money is on the second scenario.
Finding old guns in odd places isn’t limited to Europe. Here in Montana ‘relics’ are a genuine category of gun at gun shows. Someone plows a field and a rusted, barely-recognizable Sharps is unearthed…or someone finds an old shotgun stuffed in a corner of a barn…or when Grampa kicks the bucket at 97 the family finds an old .41 Remington in his dresser. Been there, done that.
The more notable cases are things like the guy who found a Thomspon gun hidden the wall of his Chicago home. Or the small-town library that finds a WW1 bring-back Maxim in their attic that used to be used for Veterans Day parades.
So..if you were going to stuff something in between the joists for some future date, what would it be?
I think that, like many folks, I’d probably breakdown an AR, wrap it up with a handful of magazines, and tuck it away with a similarly packaged Glock.
Tell you what, though…I find an StG in my attic, ain’t nobody finding out about it.