Street View nearby reveals this sign at the edge of the Street View area: "Forstarbeiter und Militär Frei," which means "Forestry Workers and Military [are free to enter]". The red circle around the sign implies that everyone else is forbidden to enter. So, it's some kind of military installation.

Neat, there's no Street View coverage but there is clear sattelite imagery: https://maps.app.goo.gl/RjGUAMExUrD6aqs59

Apple's maps version has that section blurred out though.

Bing's sattelite images seem to be older, the antenna isn't visible on there yet and there's just building foundations: https://www.bing.com/maps?cp=48.690103%7E9.086240&lvl=18.8&s.... Can't determine how old those images are though.