.emacs, this has appeared, for unknown reasons:
(custom-set-variables ; ... '(canlock-password "fdd7041be5b...")
And so on, totalling 40 characters and digits.
C-h v offers this:
canlock-password is a variable defined in `canlock.el'. Its value is "fdd7041be5b..." Documentation: Password to use when signing a Cancel-Lock or a Cancel-Key header.
I use rmail and GNUS, and those use headers, other than that it doesn't remind my of anything, really, but it can of course be something unrelated that I've overlooked.
I tried to delete it, but it came back.