That's a piece of space junk. With only 4 lum frames sigma routines aren't all that effective for removing them. A trace was left over. Sometimes I leave them in, other times I clone them out or as in this case try to use the sigma clip combine to do it. Space junk is very common. About one in 4 of my frames have a streak from space debris in them. It's getting awfully crowded up there. I keep expecting some disaster but so far we've been lucky and not lost anyone though a satellite or two has been lost to a debris hit I believe.

Rick