A boundary stone of Claudius mentioning Sergius was discovered at Rome in 1887. It records the appointment (AD 47) of the Curators of the banks and the channel of the river Tiber, one of whom was Sergius. Since Paul's journey to Cyprus is usually dated to the first half of the 40s (and some scholars would date his visit even earlier), it is thought Sergius first served his three years as Proconsul at Cyprus, then returned to Rome, where he was appointed curator. As he is not greeted in Paul's Epistle to the Romans, it is possible he died before it was written.
Some medieval legends anachronically identified him with Paul of Narbonne.
^Anthony Wagner, Pedigree and Progress, Essays in the Genealogical Interpretation of History, London, Philmore, 1975. Rutgers Alex CS4.W33., p. 59
Sources
Christian Settipani, Les Ancêtres de Charlemagne (France: Éditions Christian, 1989).
Christian Settipani, Continuite Gentilice et Continuite Familiale Dans Les Familles Senatoriales Romaines, A L'Epoque Imperiale, Mythe et Realite. Linacre, UK: Prosopographica et Genealogica, 2000. ILL. NYPL ASY (Rome) 03-983.
Anthony Wagner, Pedigree and Progress, Essays in the Genealogical Interpretation of History, London, Philmore, 1975. Rutgers Alex CS4.W33.