In article , Arthur
Figgis writes
ISTR once reading about an aircraft being sent to fly past a major
German warship, in the hope that when the ship came under attack shortly
afterwards the Kriegsmarine would think that the plane had happened to
stumble across it, rather than suspect that someone might be reading
their messages and finding them that way.

That was part of standard policy for "Ultra".

Basically, no information from Enigma decrypts could be used in the
field unless there was a plausible non-crypto-related explanation that
the Germans would believe. Of course, the information could be used to
generate that "explanation".

So, in the example I think you're thinking of, a decrypt would indicate
where and when an attack submarine was meeting with a tanker submarine
to refuel - this had to be done on the surface and neither could
submerge during the process. So a reconnaisance squadron was instructed
to send a plane out in that specific area. To their surprise, they would
find the two subs sitting there and could whistle up some bombers. If
the submariners survived the experience, they would report that they got
spotted by a plane.

--
Clive D.W. Feather