So, I'm glad I was able to figure this out because it has been bugging me all morning that I haven't been able to track this guy down. It would appear Dr. Freedman misspoke when referencing his earlier lecture, and instead of of Saint Antoninus, he meant to refer to Saint Severinus of Noricum. You can read a translation of the Life of St Severinus by Eugippius online in English. The incident Freedman is referencing is in Chapter XX:

So long as the Roman dominion lasted, soldiers were maintained in many towns at the public expense to guard the boundary wall. When this custom ceased, the squadrons of soldiers and the boundary wall were blotted out together. The troop at Batavis, however, held out. Some soldiers of this troop had gone to Italy to fetch the final pay to their comrades, and no one knew that the barbarians had slain them on the way. One day, as Saint Severinus was reading in his cell, he suddenly closed the book and began to sigh greatly and to weep. He ordered the bystanders to run out with haste to the river, which he declared was in that hour besprinkled with human blood; and straightway word was brought that the bodies of the soldiers mentioned above had been brought to land by the current of the river.

Freedman's original reference in Lecture 6:

So accommodation, improvisation. We have a saint's life that is a biography of a saint, a man named-- I'm sorry that I'm writing on the board so much today. Usually, as you know, I'm a little more in control. But these are great names. And some of them are good cats names or dog names, too. Severinus of Noricum. You know, “Stop scratching the furniture, Severinus.” That kind of thing. Severinus of Noricum. A saint in what's now, more or less, Austria. His life tells us that he learned of the end of the Roman Empire this way:

"At the time when the Roman Empire was still in existence, the soldiers of many towns were supported by public money to guard the frontier. When this arrangement ceased, the military formations were dissolved, and the frontier vanished. The garrison of Passau, which is still a town in modern Bavaria, the garrison of Passau, however, still held out. Some of the men had gone to Italy to fetch for their comrades their last payment."

This resembles a corporation-- somebody, actually, was telling me yesterday they worked for Eastern Airlines, a company that went out of business in 1990. And so sudden was the collapse of Eastern, even though it had been predicted, that she was a flight attendant and had to get on another airline in order to get home. She lived in New York; she was in Florida; Eastern ceased to exist. So these soldiers are in the same position. They want to get their last paycheck.

They were never heard from again. Nobody knew that they, in fact, were killed by barbarians on the way. "One day, when Saint Severinus was reading in his cell, he suddenly closed the book and began to sigh. The river, he said, was now red with human blood. At that moment the news arrived that the soldiers had been washed ashore by the current.”

Just for giggles, here's the Latin if you want it:

Man, that was messing with me.