If anyone recalls the name of Lieutenant-General Sir Robert Baden-Powell, most likely it's because he was the founder of the Boy Scouts (and co-founder of the Girl Scouts). But he also worked as a spy for the British Secret Intelligence Service, as described and illustrated in his book My Adventures as a Spy (London: C. Arthur Pearson, 1915). No longer under copyright, it is available online to download as a pdf.
In one instance, he pretended to be a lepidopterist in what was then Dalmatia (now Croatia). Walking along with a net and sketchbook, he positioned himself on hills overlooking strategic fortresses and appeared to make drawings of butterflies, one of which is shown above. In truth, within those drawings, he was concealing cryptic diagrams (camouflaged or embedded figures) of the structures and their armaments.
He recommended such activities. Indeed, as he states in his book, "…for anyone who's tired of life, the thrilling life of a spy should be the very finest recuperator!"