Two Norfolk lifeboats and a helicopter were called to an area off the county's coast following reports of an upturned boat in the water.
Lifeboats from Caister and Gorleston were called to the incident 13 miles from the coast off Haisborough Sands at 11.45am on Thursday (November 17).
Crews remained at the scene for almost five hours but after an extensive search found no evidence of people in the water.
Guy Gibson, the coxswain for Caister Lifeboat, said: "When we arrived on scene there was an inflatable boat which was about 25ft-30ft long.
"The helicopter reported it was upturned but we discovered it the right way up.
"As the boat was empty, we were tasked with a search pattern which took several hours."
It had been reported the boat could have been used to smuggle migrants into the UK and had been left abandoned in the sea.
Mr Gibson said he could not confirm whether it was migrants and said it was "inconclusive".
The boat was lifted out of the water onto a rigged standby vessel before crews returned to the station at 4.30pm.
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