question:  Why does Paul think an alien wouldn't be able to hide on Earth?
context: One can't be too cautious about the people one meets in Tangier. They're all weirdies of one kind or another. It marks the end of Boulevard Pasteur, the main drag of the westernized part of the city, and the beginning of Rue de la Liberté, which leads down to the Grand Socco and the medina. King-size sidewalk cafes occupy three of the strategic corners on the Place de France. Tangier is possibly the most cosmopolitan city in the world. In native costume you'll see Berber and Rif, Arab and Blue Man, and occasionally a Senegalese from further south. In European dress you'll see Japs and Chinese, Hindus and Turks, Levantines and Filipinos, North Americans and South Americans, and, of course, even Europeans—from both sides of the Curtain. In Tangier you'll find some of the world's poorest and some of the richest. The poorest will try to sell you anything from a shoeshine to their not very lily-white bodies, and the richest will avoid your eyes, afraid you might try to sell them something. As a result of them the permanent population includes smugglers and black-marketeers, fugitives from justice and international con men, espionage and counter-espionage agents, homosexuals, nymphomaniacs, alcoholics, drug addicts, displaced persons, ex-royalty, and subversives of every flavor. Local law limits the activities of few of these. Paul said, "What ever happened to those poxy flying saucers?" A French girl went by with a poodle so finely clipped as to look as though it'd been shaven. "You know, what everybody was seeing a few years ago. I didn't know Paul very well, but, for that matter, it's comparatively seldom you ever get to know anybody very well in Tangier. Largely, cards are played close to the chest. Tapas at the Cafe de Paris are apt to be potato salad, a few anchovies, olives, and possibly some cheese.
output:  The Earth has so many intelligence agencies, at least one would be watching when an alien gave itself away.
options:  Aliens don't look like Earthlings.  An alien would not be able to mimic a human enough to fit in with society.  An alien wouldn't be able to assimilate into Earth's backward culture.  The Earth has so many intelligence agencies, at least one would be watching when an alien gave itself away.