Zzzap can see ships’ magnetic signatures

“Are you certain?” asked Stealth.
Barry looked at her blank mask. “There’s a man-made island out there? Yeah, of course.”
“Of the location.”
“Oh.” He swallowed another mouthful of food. “Pretty sure, yeah. I mean, there aren’t any landmarks. I’m going off magnetic flux lines in the atmosphere. It’s maybe seventeen or eighteen hundred miles from here, south-southwest. I saw the magnetic signature on the water from a few miles away and doubled back to check it out.”
Richard looked up from the map. “Magnetic signature?”
Barry’s bald head went up and down. “It’s a huge chunk of metal, so it makes ripples in the Earth’s magnetic field. Nothing huge. It stood out because it was in the middle of the ocean on top of the water.”

Excerpt From Ex-Isle, pg 69
Peter Clines
https://books.apple.com/us/book/ex-isle/id989619017