I've seen the section in the manual on all the different shapes that the crosshair can take, but I've noticed that it sometimes also gets an arch or "eyebrow" above it. I haven't seen in the manual, or been able to deduce the meaning of that.

Anyone know?

The 'arch' or 'eyebrow' appears associated with other characters once they take some damage. It gives you a rough estimate of how much of their 'life bar' is gone.

Yeah, it's the health bar of someone who is either on your side or an enemy. You'll see it if you see a guard fighting something or someone, or the people who are with you during a quest, etc.

How does it demonstrate the amount of life left? Does the length vary?

Yes it does.

Cool. Thanks all.