During WW2 the British (and allies) occasionally managed to capture an Enigma machine and/or code books from a sinking sub or weather-ship.
How did they keep the capture secret?
The crews must have known that the codes hadn't gone down with the ship, and are likely to have seen the British boarding parties going on-board. As POWs, their family would have been informed of their survival (I think the Geneva convention requires this?).
Didn't the value of the captured codebooks depend on the Germans believing they'd gone down with the ship? If they knew that the crew had survived wouldn't they think the vessel had survived and change the codes as a precaution? Or did the British keep these survivors isolated?
I'm currently reading Hugh Sebag-Montefiore's enigma - the battle for the code' it mentions the captured codebooks, but not how the German sailors were kept silent, I know they were sent to a POW camp which included Germans who definitely were allowed to write home.
UPDATE - According to Wikipedia (so it must be true!) Operation primrose which involved the capture of codebooks and 37 German submariners was the 'biggest allied secret for 7 months of the war'
I guess my question really boils down to, did the allies admit that they'd captured the submariners (which is what's supposed to happen) or did they hold them completely isolated from other POWs and only allow them into general circulation 7 months later? Or were they so confident in their censoring of the mail that they felt that they could tell the families they were still alive, allow them to mingle with fellow prisoners and somehow still be confident the information couldn't leak back?