Assuming you had only three men (two crew + WO) on the conning tower: watch their routine and realize that two crew only scan 180° of the horizon. Every crew member scans only a 90° sector. So only with all four positions manned the complete 360° are covered. Additionally the WO scans the full 360° circle, but he freezes occasionally - especially after a requested report. This might have caused your problem. The PBY coming out of an unobserved sector and the WO staring somewhere. Man all conning positions when surfaced!
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