I finally found out what these little signs are:
A friend in the flying club asked his other German friends to discover what these signs are. It seems nobody knows, because it's only something that the utility people use to find out specifics about the sewage, water, or gas lines. In German, they're called
Hinweisschilder zu Straßeneinbauten, literally translated to "Signs to road installations" The Wikipedia page only has the description in German.