German news weekly Stern reported on Wednesday that invasive employee monitoring is common practice at other retailers, including Penny, Netto, Norma, Edeka, Plus, REWE, Hagebau and Famila. As was the case with Lidl, the retailers routinely printed up pages of notes detailing intimate aspects of their employee's lives.

The magazine said that, nationwide, detective agencies have sprung up which specialize in helping employers spy on their staff, including "detailed written observation reports."

Stern quoted the manager of a supermarket discounter who said that the detective agencies are extremely competitive.

"Many try to set themselves apart by offering additional services," the manager was quoted as saying. "I was offered such protocols more times than I can count."

The manager confirmed that spying efforts were intensified when there were suspicions that certain employees were attempting to organize a union.

"Every measure is used in order to prevent a works council from forming," the source told Stern.

Workers outside the retail sector also have reason to fear for their privacy. According to a study by Mummert Consulting, every third office computer in Germany is subject to some sort of monitoring.

Germany's federal association of detectives also admitted that 60 to 70 percent of their contracts come from companies, and that the most common request is for the detective to observe employee behavior.