"What's to hinder a motor-car being fitted with secret aerials?" he asked. "The Germans are not children. They're up to all sorts of cunning tricks. Why, only last week one of our Haddisport drifters went out to the herring fishing with a splash of red paint on her starboard bow. Nobody knew who put it there, the crew least of all; they didn't even see it. But when the boats were drifting to their nets on the fishing ground, a German submarine came nosing round, spotted the red splash of paint, and then went off in a bee-line for Heligoland."
"Well?" questioned Vera, not understanding. "What did it mean?"
"Well?" repeated Mark. "I don't know what it meant. But the men on the submarine did. It was a pre-arranged sign—a message. It's an old Scout trick. Darby Catchpole wanted to communicate with me once, by a way we'd fixed upon. I watched for the postman, and when he came past this gate I saw some flour dust on his left arm. That meant 'No.' If the flour dust had been on his right arm, it would have meant 'Yes.' In the same way a German spy could put a secret mark on a railway carriage or a motor-car, going to a known destination, and give information to hundreds of other spies along the route."
"Dear me!" exclaimed Vera. "Perhaps it was a spy who tied the mysterious piece of ribbon to the handle-bar of my bicycle yesterday!"
"Likely enough," surmised Mark. "Perhaps the same one who daubed the paint on that fishing-boat. There's no doubt there are spies around here. And there's a green motor-car that goes dashing about between here and Buremouth with lamps shining like searchlights. The police and the military patrols have had instructions to capture it. Constable Challis has been put on night duty now. Challis is rather too fond of talking, but he's an uncommonly smart policeman."
Mark Redisham's estimate of Constable Challis was justified sooner than he expected.
On the very next night, indeed, Challis was on his beat patrolling the rabbit warren and the dark lanes to the north of the town, when his smartness was put to the test.
Formerly he would have been watching for tramps, suspicious loiterers, and possible burglars; but, since the outbreak of war, crime had diminished, even gipsies were fewer, and he could do nothing so useful as to watch the road for unauthorised vehicles and for spies flashing signals across the sea.
Before ten o'clock he had visited five different houses to alarm the occupants by informing them that lights were visible from their windows.
In three cases it was discovered that the lights were to be seen through the chinks of imperfectly drawn curtains or ill-fitting blinds; in one case a nurse had left the gas burning by mistake, and in the other, where the light came through an open stable door, a groom was attending to a sick horse and had not known of the new regulations. By midnight, however, the whole neighbourhood was in darkness.