 Why does Paul think an alien wouldn't be able to hide on Earth?
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?" You're not eligible for membership until you, well, say until you've developed space flight.
 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.