BDMLR said the animal had been “free swimming” in a small space in front of a bridge in Gweek village when they arrived on Monday.
“We had begun to plan for an intervention, however, it then swam off downstream and made it as far as the Cornish Seal Sanctuary site before turning round and heading back in,” it said.
“It took us a little while to find it again but we found it over the far side of Gweek boatyard just as it got dark.
“At that point, it was high tide and there was no reasonable course of action we could take in those circumstances, so we stood down and awaited reports from local residents and sanctuary staff who would watch out for it again first thing the next morning.”