What is this golden streak artifact in this photo?

Question

This was taken at 1/500sec, iso64, f/2.8, 122mm focal length. Nikon D850, 70-200mm lens.

At first I thought it was her hair, but it seems to be very reflective and moving very fast:

At 100%

It even seems to change direction at the bottom. If it is at the subject's distance, that's moving, say, a foot in 1/500s. Or over 300mph. It's seems out of focus, so probably closer to the camera. It seems to change speed since the brightness of the streak changes?

Is this not a streak and just a fiber on the sensor? I did not see this in any other photos.

Answer 1

There's a saying that's taught to doctors during their training regarding diagnosis:

When you hear hooves, think horses, not zebras

The point is that, unless you're in situations where you might expect the exceptions (i.e., on safari in Africa where you'll probably see zebras), it's likely that the simplest explanation is probably what's going on.

In this case, it's likely a hair, probably from the woman on the right. Addressing some of the reasons that lead us to think it's not a hair:

1. Too big to be a hair: it's not at the plane of focus; either somewhat in front or behind the plane of focus. The hair appears "wider" due to the lens's bokeh characteristic.

2. Turns too sharp angles: don't forget the 3rd dimension / z-axis. The "figure-8 loop" at the end only looks like a tight loop when viewed along the lens's axis. In fact, the hair could be looping by several inches (or even feet) directly towards or away from the lens at the top & bottom of the loops. The only rational indications we have that it doesn't loop several feet is that the rest of the woman's hair isn't that long, and that the loop extents aren't a complete blur (meaning, the amount of defocus "spread" of the hair limits how far in front of or behind the plane of focus the hair goes).

Answer 2

It has to have been relatively static in-frame. It cannot have been a single moving point.

As already mentioned in your question, for a single bright point to show as a line covering maybe 1 foot distance, it would not only have had to be incredibly bright, but also travelling at about 350 MPH & at an almost constant speed, or the brightness in the photo would vary more than would be allowed for in the two slightly dimmer changes of angle at the half-way point. I'm pretty certain no insect could pull that off, including the complex turn at the bottom without slowing down.

Therefore, it must have been relatively static. Too coarse for a hair [or it's much closer than we think], fishing line could fit the bill. Hard to tell whether it's really in front or behind the subject [122mm is going to flatten our depth perception a lot], but it's not in the same plane as the bucket handle.
It may have been attached/stuck to the subject, or disturbed briefly by movement or wind - visible only as it hit just the right angle to flare in the light. The slight brightness changes would fit the reflectivity of a hair-like object as it changes angle in the light.