Do you know of any court case testing this?
My contract with a German company, specifically had clauses saying that anything on their systems is something that can inspect and/or look at.
We aren't talking a small company here, but one with multiple large sites; which means multiple large workers councils ( basically a union for a specific company location ).
Remember hiring in Germany has to be approved by a Worker's Council and the company has to pay for the council to receive independent legal advice as well.