The magazine refers to more than 100 pages of internal documents from the store, which it says reveal that the chain uses miniature cameras that are officially meant to provide branch security to record workers on the job.
Logs record the day and hour that employees use the restroom, with whom they have possible love interests, and which employees seem incapable or just "seem introverted and naive," the magazine reports.
The logs Stern refers to are from the German states of Lower Saxony, Rhineland-Palatinate, Berlin and Schleswig-Holstein.
Lidl confirmed the existence of the logs with Stern, saying that they aren't used for "employee surveillance, but the identification of possible misconduct."
The chain distanced itself from the surveillance of their employees' private lives, explaining that "details and observations do not correspond to the way we deal with our employees."
Germany's government official for data protection Peter Schaar told the magazine that logging toilet visits and other things was a breach of German data protection law. "I take it that if such procedures become known, the appropriate authorities will investigate," Schaar said.