Being from Germany myself I know the problem - but have the impression that it usually happens in large cities that were bombed during the final stages of the war. Unlikely that there would be many bombs left in a forest - the site is far enough outside of Berlin to be affected IMHO.
Three Germans discussing in this US forum all the ammunition the Air Force dropped in WWII around Berlin and lef not exploding
Seriously a small not really nice forest will be cut down to build GF4 and Elon already said the will reforest it but it would be a big surprise if not at least one Bomb will be found on that ground maybe more as well as all the weapons soldiers did throw away in forests and lakes in the last days of that tragic disaster.