I have to agree with the others: I highly doubt that the periscope was, itself, spotted, as much as 1) it's feather (the spray kicked up, much like a bow wave) and 2) the wake it was leaving behind.
You very very rarely see any straight lines in nature. The weather was good and still quite a bit of daylight. With only 2 meter winds the sea was quite calm. Depending on how long you had the scope up you were likely kicking up and leaving quite a long, straight wake, especially at 6 knots. The merchie crew might not know exactly what they'd seen, or known for sure it was a U-boat. But zigzagging for half an hour beats just dumbly stooging along in a straight line and risk becoming easy meat.
Best thing to do under those circumstances is drop your speed to ahead slow or all stop, wait until you slow or stop, then raise your scope and take your bearings, ranges, speeds, etc. Then lower the scope and speed up again. That way you won't be spotted as easily.
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