M-x grep
grep  -nH --null -i -e header-line *.el /dev/null

-*- mode: grep; default-directory: "~/.emacs.d/emacs/lisp/" -*-
Grep started at Sat Apr 19 14:09:03

bindings.el:572:    (define-key map [header-line down-mouse-3]
mode-menu-binding)

Right here is a null char: bindings.el^@:572 (it is not pasteable into
email).

-Stephane