Both Penarth lifeboats and Coastguard Rescue Helicopter 187 were involved a search of the coastline from Lavernock Point to the Cardiff Barrage last night.
The helicopter, using a powerful searchlight, infra-red heat-detecting equipment and night-vision goggles – carried out several passes along the shoreline at an altitude of 300 feet.
Both local lifeboats also carried out co-ordinated searches – with the smaller D Class vessel working close to the shore on the evening’s high tide.
Penarth Coastguard officers were also on the scene liaising with local police and conducting a shoreline search.
It’s understood the alarm had been raised by an eyewitness on the Cardiff Bay Barrage but nothing was found.