Nobody flashed a shield at the Battle of Marathon, for it is scientifically impossible: hoplite shields were curved, and you cannot reflect a flash from a curved surface. The shield can only have been waved (as Herodotus says), making it a short range signal, not from far-off traitors in Athens but traitors at Marathon, signalling the movements of their own army to the enemy. The Persians' voyage round Sounion was long and tedious, and the quickest way to Athens would be to land as soon as possible after Marathon, at Loutsa, and for the cavalry to dash for the city. The signal warned that this route was blocked, hence Plan B—Sounion.

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Journal Annual of the British School at Athens
Hodge, A.T. (2001). Reflections on the shield at marathon. Annual of the British School at Athens, 96, 237–259. doi:10.1017/S0068245400005281