I'm reading Late Antiquity: A Guide to the Postclassical World. On page 114, when they are discussing treatment of barbarian tribes when captured by the Romans and spread throughout the Empire, it says "One such unit of Franks sent to the shores of the Black Sea managed a heroic escape, commandeering a ship and making their way across the Mediterranean, through the Straits of Gibraltar and ultimately home..." (here in Google Books).
When exactly did this happen? And what historical sources talk about it? Are their any secondary sources covering this as well?
These events are recounted in what are known as the Panegyrici Latini, a written collection of anonymous orations describing the reigns of various Roman Emperors in the late 3rd and early 4th centuries. The expedition of these daring Franks is described as happening in the reign of Probus (r.276-282). The captives are said to have hijacked some ships in the Black Sea and made their way into the Mediterranean, seemingly terrorizing coastal communities with impunity. They apparently did a great deal of damage to the Libyan coast and even sacked Syracuse at one point. They then continued west and through the Straits of Gibraltar and eventually returned to their homeland.
The episode is also recounted in the New History, a work by a Byzantine historian named Zosimus (late 5th early 6th century). His version of events is pretty much the same but he does mention a battle between the Franks and a detachment of men from Carthage that stopped the brigands from causing further harm on the African coast. Even so they seem to have made it through the Straits without serious loss. The fact that the Franks were able to pull this off speaks to the chaos afflicting the empire in the 3rd century.
See here for the Panegyrici Latini and here for Zosimus. A good secondary source that mentions it (albeit briefly) is Constantine and Eusebius by Timothy David Barnes.