It may be that many persons who owned airguns thought they had better get rid of the airguns rather than be found in possession of what looked like a real weapon and risk arrest or whatever.
If substantial training airguns were made for the armed forces, then I would think that they were kept in barracks and duly destroyed when Germany was over-run or perhaps they had already been melted and machined into powder burners when materials became scarce in the closing stages of the war.
I am no historian but just guessing.