The Spy Paramount by E. Phillips Oppenheim

Fawley, after several weeks of devious and strenuous wanderings, crossed the very fine hall of Berlin's most famous hotel, well aware that he was now approaching the crucial point of his enterprise. Frankfurt, thanks to his English and French connections, had been easy. At Cologne and some of the smaller towns around, even if he had aroused a little suspicion, he had learnt all that he needed to know. But in Berlin, for the first time, outside aid was denied to him and he became conscious that he was up against a powerful and well-conducted system of espionage. The very politeness of the hotel officials, their casual glance at his credentials, their meticulous care as to his comfort--all these things had seemed to him to possess a sinister undernote. He chose for his headquarters a small suite upon the sixth floor, with the sitting room between his bedroom and bathroom; but his first discovery was that the one set of keys attached to the double doors was missing and he only obtained the keys giving access to the corridor after some considerable delay.

Yet to all appearance he had been received as an ordinary and welcome visitor. According to his custom, he was travelling under his own passport and without any sort of compromising papers, yet all the time he fancied that these polite officials, some of whom seemed to be always in the background, were looking at him from behind that masked expression of courtesy and affability with definite suspicion.