Here's an explanation for phantom airships which I haven't come across before: whales!

The way in which rumours start and grow is shown by the following incident recorded by the Daily Telegraph correspondent at Harwich:—

"It was rumoured in Harwich this evening that a Zeppelin had been seen flying on the North Sea to-day, surrounded by British destroyers. The story was brought into this port by members of the crew of the Great Eastern Railway Company's steamer Colchester, which arrived late in the afternoon from Rotterdam. On enquiry I have ascertained that when within twenty-five miles of Harwich the crew of the Colchester saw a large object of a yellowish tint afloat on the water, with two destroyers near by. The weather was hazy, and it was difficult at a distance to determine precisely what the object was. One of the destroyers fired at it; the other steamed away. The true explanation of the incident is now stated in naval circles to be that the supposed Zeppelin was merely a dead whale, and that the carcase was fired at with the object of sinking it.
"'Did it look like a whale?' I asked a member of the steamer's crew.
"'Oh, yes, it might have been,' he answered."

Source: Flight, 23 October 1914, 1065 (link).

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