From How the Cadillac Got Its Fins by Jack Mingo:

When World War II started, the company built 168 blimps for observation and antisubmarine missions. They patrolled both coasts and accompanied naval vessels. One blimp was the center of a mystery that has never been solved.

In the early hours of August 16, 1942, the airship Ranger took off from Treasure Island in San Francisco Bay with a crew of two. It carried two antisubmarine depth bombs. That afternoon, residents of Daly City, down the peninsula from San Francisco, were surprised to see a blimp coming in for a perfect landing in the middle of a residential street.

The blimp was well-stocked with fuel and helium, its batteries were charged, the radio was operating, and emergency life raft and parachutes were on board. Everything seemed to be fine, except for a few small details: The door was open; one depth charge was gone; and both members of the crew were missing. Neither man was ever seen again.

The navy never came up with a satisfactory explanation.