 Why does Paul think an alien wouldn't be able to hide on Earth?
In native costume you'll see Berber and Rif, Arab and Blue Man, and occasionally a Senegalese from further south. 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?"
 The Earth has so many intelligence agencies, at least one would be watching when an alien gave itself away.
 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.