Detection by happenstance

In such cases, when he could not follow the train of the reasonable, he coldly and carefully followed the train of the unreasonable. Instead of going to the right places – banks, police stations, rendezvous – he systematically went to the wrong places; knocked at every empty house, turned down every cul de sac, went up every lane blocked with rubbish, went round every crescent that led him uselessly out of the way. He defended this crazy course quite logically. He said that if one had a clue this was the worst way; but if one had no clue at all it was the best, because there was just the chance that any oddity that caught the eye of the pursuer might be the same that had caught the eye of the pursued. Somewhere a man must begin, and it had better be just where another man might stop.

Chesterton, The Blue Cross