U 377 (Kapitänleutnant Gerhard Kluth) had left Brest into the North Atlantic on 15 December 1943 for its twelfth operational patrol. On 23 December 1943 U-boat Command directed the boat to join group Borkum stationed to the west of Spain across the convoy route between Britain and Gibraltar. After conducting operations against various Allied convoys and A/S groups passing through its operational area during the following weeks, group Borkum was finally dissolved on 13 January 1944. At 1232 on the same day U 377 was attributed to group Rügen, then operating to the west of Britain in a loose formation across the North Atlantic convoy routes. Together with other ex-Borkum boats U 377 was to extend the patrol line towards the south. Its new operational area covered a radius of 30 nautical miles around naval grid square BE 2144. By the same signal U 377 was ordered to change to 'Irland' radio service as from 0800 on 14 January. At 2206 on 14 January group Rügen was partly rearranged with U 377 now assigned to naval grid square AL 9798. U-boat Command advised the boats to use economical speed and to move submerged during daylight. At 1605 on 19 January group Rügen was again ordered to shift into new positions by 1900 on the next day with U 377 assigned to AM 4777. The operational radius was now reduced to 20 nautical miles. After unsuccessful operations group Rügen was eventually dissolved on 26 January and at 1958 on the same day U 377 was reassigned to the newly formed group Hinein. This group, consisting of eight boats, was ordered to form a patrol line from 1730 on 27 January stretching from AM 7132 to AM 7628. U 377 was to occupy the position AM 7259. U 377 reported last at 0404 on 15 January 1944 from square BE 5748 about two attacks with homing torpedoes against an unidentified search group, claiming to have heard a heavy explosion after 9 minutes following the second shot without being depth charged thereafter. U-boat Command plotted the boat to have commenced its return trip to France on or about 29 January 1944. When U 377 thereafter failed to enter La Pallice as the newly assigned port of arrival by 10 February 1944, U 377 was posted as missing the same day with effect from 4 February 1944. In the absence of any information about its fate, U-boat Command considered the possibility that U 377 was already lost in January 1944 during operations with groups Rügen or Hinein.

After the war the Allied Assessment Committee nevertheless felt unable to attribute the loss of U 377 to any known A/S-attack carried out in the eastern North Atlantic or the Bay of Biscay during the period when the boat supposedly disappeared. Therefore the loss of U 377 remained unexplained and the boat was officially recorded as 'lost to unknown cause'.

U 641 (Kapitänleutnant Horst Rendtel) had left St. Nazaire in France on 11 December 1943 for its fourth patrol in the North Atlantic. At 1424 on 18 December 1943 U-boat Command directed the boat to join the newly formed group Borkum stationed to the west of Spain across the convoy route between Britain and Gibraltar. After conducting operations against various Allied convoys and A/S groups passing through its operational area during the following weeks, group Borkum was finally dissolved on 13 January 1944. At 1232 on the same day U 641 was attributed to the group Rügen, then operating to the west of Britain in a loose formation across the North Atlantic convoy routes. Together with other ex-Borkum boats U 641 was to extend the patrol line towards the south. Its new operational area covered a radius of 30 nautical miles around naval grid square BE 2515. By the same signal U 641 was ordered to change to 'Irland' radio service as from 0800 on 14 January. Following a request by U-boat Command at 2206/14 U 641 radioed in its last known signal at 0318 on 15 January that the boat intended to commence its return to France on 20 January 1944. When the boat thereafter failed to arrive in port by 2 February 1944, U-boat Command ordered the boat at 1215 on the same day to report its position forthwith. In the absence of any reply U 641 was posted as missing on 3 February with effect from 2 February 1944.

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