The Master Spy by Arthur Gask — Read

Larose wasted no time in taking in the scanty furnishings of the waiting room, but tip-toed quickly through a half-opened door, into what, he knew at once, must be the consulting room, because of the desk, the microscope and the row of dusty test-tubes in a rack upon the mantel-shelf.

But for the moment he gave only a swift glance round, and then he darted to another door that he saw led into a much smaller room. Then, with noiseless steps, he was back again in the waiting-room and finding out where the fourth room led.