L-8 Blimp; Wikimedia Commons
A few hours after leaving for a routine anti-submarine patrol out of San Francisco in August 1942, the U.S. Navy blimp L-8 drifted back empty.
An hour into the patrol the pilot had radioed that they were about to examine an oil slick on the ocean. When it landed, rescuers found the gondola door propped open and the engine set on idle. Two life jackets were missing, but the pilot and co-pilot were never found.