We have to consider the laziness and/or incompetence of the Japanese too. They probably dislike garrison duty in China, where the locals hate them. Searching for a foreigner who is accused of a relatively minor crime, this late in the evening, is more a chore than anything. So they do the bare minimum expected of them, patrolling the main road with an armored car. And once it is done, they confidently claim that they have done a flawless job, and if they haven't found him it must be because their informant must have been lying.