The Landesamt für Denkmalpflege Baden-Württemberg just published a leaflet with guidelines on how to deal with metal detectorists or people carrying out illegal excavations. It includes the definitions of Raubgräber (someone excavating without permission) and Sondengänger (metal detectorists), and the current legal background in Baden-Württemberg. It is in any case suggested to call the police in case you see someone carrying out such activities.

You can download the guideline here.