A red distress flare sparked a full-scale search and rescue operation.
The rocket was seen between Holme and Thornham at around 4pm on Sunday.
Hunstanton RNLI lifeboat was launched, while Hunstanton and Wells coastguard response teams were also called out.
The teams searched from Brancaster to Holme, finding nothing. They believe the flare to have been fired in the Holme to Thornham marsh area. All teams were stood down at 6pm.
The RNLI said: "Red flares are international distress signals at sea. They should not be used indiscriminately, and never in coastal areas ashore."
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