A major search operation involving hundreds of officers is under way for the missing baby of aristocrat Constance Marten and her partner Mark Gordon after the pair were arrested in Brighton on Monday night.The pair were located by officers from Sussex Police in Stanmer Villas, Brighton after a member of the public reported seeing them shortly before 9.30pm. The baby was not located with them.They have been arrested on suspicion of child neglect after 54 days on the run.