A Whispering of Spies by Rosemary Rowe in the Libertus series.
A gruesome discovery leads Libertus on a dangerous quest . . . Wealthy Volus, ex-lictor to the Imperial Governor of Gaul, is retiring to the town of Glevum. Libertus is sent to his new apartment, where he is informed that one of the ex-lictor’s treasure carts has been intercepted, the guards and horses brutally butchered. When his actions are misinterpreted by a network of spies, Libertus is suspected of involvement in the massacre and marched to the garrison to await trial. But after daringly escaping, Libertus embarks on a dangerous quest to discover the truth . . .