I think it was something he actually cultivated, for cutting throough
ropes when tied.
I remember reading a Max Brand western where the hero is captured in
chapter 2 and thrown in jail. Then we find out he routinely keeps
lockpicks taped to the back of his neck, under the hairline.
Like the line in Wodehouse.
"Oh, we're really in the soup now."
"Well, we would be, if I hadn't taken the sensibile routine business
precaution of signing a false name to the lease."
dave