As Picus said there was no breeze, the distress flag would have hung
limp against the mast, and the Captain could not have seen, at that
distance, whether or not the flag was upside down.
That’s all the Professor needed to determine the falsity of his alibi.
However, Picus was a poor sailor. While the International Distress
Signal is a flag flown upside down, it is by custom and regulation
always flown at half-mast.
... And the sea charm’d into a calm so still
That not a wrinkle ruffles her smooth face.
Dryden.