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).