More detail of the 1904 incident.
"In 1903-1904 French intelligence obtained for 60,000 francs a set of
documents outlining the developing Schlieffen Plan: a massive strike through
Belgium. The circumstances of their delivery invited suspicion. "Le Vengeur"
was presumably an officer of the German General Staff. But he kept his head
swathed in bandages to prevent identification. Only his "Prussian" mustache
was exposed, presumably as proof of his national origins! He had previously furnished useful information. But had that merely been a preliminary to a
massive disinformation plan? The "Vengeur" documents ultimately proved less
useful for themselves than as a spur to study German railway construction along
its Belgian frontier, and the writing of such contemporary military theorists as
Friedrich von Bernhardi. Both of these led French military intelligence to
predict a German invasion through Belgium and Luxemburg even before
Schlieffen himself had made his final decision.22

http://www.dtic.mil/dtic/tr/fulltext/u2/a433346.pdf