I remember reading somewhere that there were two Type XXI's in a sealed-up U -bunker somewhere - possibly near Hamburg. If memory serves, the bunker was sealed up with concrete after a young man was exploring the bunker and fell, and ended up getting himself killed.
edit - Ah yes, here it is - the Elbe II Bunker THREE Type XXI's are down there, still.
http://www.uboat.net/history/hamburg_elbe2.htm
http://www.uboat.net/flotillas/bases...rg_bunkers.htm