Good question antidotos. We had gone to great lengths to attain information on where the major allied minefields were and when they were there. The minefields are as accurate as can be determined, and in no cases are there any that did not exist
U-12, U-16 & U-40 were lost in October 1939 in the Dover Straights.
German U-Boat Losses During World War II - Details of Destruction by Axel Niestlé says that there were only 7 to sea mines. Where as U-boat net says there were 35. On top of these figures there are 72 listed as missing.
Anyway, the mines will be making an appearence in a enemy harbour near you soon!