From what you wrote, it does not look like your relative was in a German POW camp. As you say there were Stalags (for enlisted personnel) and Oflags for officers. Greater Germany was divided into military districts known as Wehrkreise. Each one had a number of POW camps. For instance, WK XII Wiesbaden had Oflag XIIB (Hadamar), Stalag XIIA (Limburg), XIID (Trier) and XIIF (Forbach) in 1944.
Roger