During a time when very few humans would have had the presence of mind or equipment to detect such an event, one species faithfully recorded this freak occurrence for our puzzlement millenniums later: Japanese ceder trees. The evidence of this event is told by the pattern of tree rings in this species, which shows a huge anomaly right around this time in history. There is a massive amount of activity from carbon-14, an element only presented when massive amounts of radiation from space bombard earth's atmosphere.
Sometime around 774 or 775 A.D., a mysterious burst of radiation hit the Earth. We have no idea what caused it, where it came from, or why it went otherwise undetected.
A supernova would be a possible explanation, but early astronomers show no record of such an event visible from Earth during this time, despite having noted supernovas in 1006 and 1054. For that matter, we'd still be able to find evidence of a supernova from this time today. A solar flare could also explain it, but again no solar flare of such a large and devastating nature would have come and gone without somebody noticing it. If you're picking up the theme here, huge bursts of radiation have to be caused by a star or something as big and hot as a star.
Other tree species around the world, as well as ice core samples from the polar regions, bear a similar record about the same time period in history. So either we had an invisible, undetectable solar flare or supernova, or we have a bunch of lying trees on our hands.