Footprints Under the Window (The Hardy Boys, #12)

It starts with the two heroes rescuing a stowaway and his strange utterance about "footprints." But he flees. It appears to be an espionage ring, but they can't be certain. At one point the teens, along with pilot Jack, follow the clues to a group of islands off the coast of South America!

Librarian's note: this is an introduction to a later edition. The original 1933 edition under the same title relates to a Chinese laundry in Bayport and human smuggling - with racist overtones.