I've had the grey icon when it's been a clean kill, but others are right when they say it's normally attributed to sinking caused by bad weather. As far as I'm aware I did get the credit for the sinking as it appeared in my log book.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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