The "Pax Romana" is a period spanning a couple centuries and quite a few emperors, it's not exactly the most precise contextualization. I understand what you mean, though, so all's well. By Roman law theft, furtum, was generally the subject of a civil suit rather than a criminal action, although certain types of theft were considered capital crimes and by late antiquity both civil and criminal action could be taken on theft. Provided that there was no breaking-and-entering (which under the Principate was considered crimen) and that these were private funds being stolen in most cases the victim brought a civil suit against the thief. If it could be proven in court that theft had occurred the penalty might vary. For civil suits involving theft the old punishments of the Twelve Tables (which included flogging and possibly death at the hands of the victim) were superseded by the Principate with remuneration of the goods stolen, at a double rate in the case of furtum nec manifestum (theft that was not readily obvious and had to be proven in court) and at a quadruple rate for cases of furtum manifestum (obvious theft--usually if the thief was caught in the act)