there's always a lot of people who hear things they oughtn't to.
What surprised him was that it got so quickly to the enemy.

Then after last February, when the Hun submarines went in for
frightfulness on a big scale, the thing grew desperate. Leakages
occurred every week, and the business was managed by people who
knew their way about, for they avoided all the traps set for them,
and when bogus news was released on purpose, they never sent it.
A convoy which had been kept a deadly secret would be attacked at
the one place where it was helpless. A carefully prepared defensive
plan would be checkmated before it could be tried. Blenkiron said
that there was no evidence that a single brain was behind it all, for
there was no similarity in the cases, but he had a strong impression
all the time that it was the work of one man. We managed to close
some of the bolt-holes, but we couldn't put our hands near the big ones.
'By this time,' said he, 'I reckoned I was about ready to change
my methods. I had been working by what the highbrows call
induction, trying to argue up from the deeds to the doer. Now I
tried a new lay, which was to calculate down from the doer to the
deeds. They call it deduction. I opined that somewhere in this
island was a gentleman whom we will call Mr X, and that, pursuing
the line of business he did, he must have certain characteristics. I
considered very carefully just what sort of personage he must be. I
had noticed that his device was apparently the Double Bluff. That is
to say, when he had two courses open to him, A and B, he pretended
he was going to take B, and so got us guessing that he would try A.
Then he took B after all. So I reckoned that his camouflage must
correspond to this little idiosyncrasy. Being a Boche agent, he
wouldn't pretend to be a hearty patriot, an honest old blood-and-
bones Tory. That would be only the Single Bluff. I considered that