I saw trails like this through the spotty when my partner was shooting in cold weather when the temperature was very close to the dewpoint. We were using normal 7.62x51 ammo and shooting 600-1000 yds. It was during a class with FARGO, whom I haven't seen here for a long time. He said he'd seen it before in similar conditions.
When I was competing it was called trace. A good spotter could call the shot and verify when the target came back up.