I have full Sturdy Combat Armor set and a Guess rifle. I was thinking it would be cool to see my supply line settler one shotting things that attack her, but I can't find her anywhere. Does she ever reappear?

She's walking around and you can't find her.

And you can be sure that she'll be fighting some enemies around.

I'm sure I've seen them going back and forth. Someone here said they put miners helmets on them so they could spot them easily at night.

They're pretty easy to spot in gear that differs from other settlers (e.g., I sometimes put mine in Spike Armor or other less common but sturdy outfits) but they're pretty easy to spot even without doing anything special due to the Brahmin following them everywhere (except when it gets stuck somewhere... but it eventually will catch up, anyway).

But yes, you can see them fighting and even follow them on their routes so that you can deal with encounters as a pair rather than alone.

And yes, they can die, too.

How do I find her?

I put random outfits and hats on mine. Is it true that they wont ever die? That's something I've always been curious about. Some say yes and some say no.....

They are easy to spot in that an pack bramin is following them and they are named provisjoner.

If you have hubs for your suply lines you will see them all the time, they make an traffic jam in the castle doorway for me.

Recommend to arm for bear as they can into fights if they are close to you, they can die happened a couple of times for me.

Combat armor and combat rifles or better.

They might be protected like followers in Skyrim, they will not be killed by enemies shooting at them but if knocked down they might be killed by explosions so if an raider shoots them and knock down and another throws an grenade they might die. This require that they are close enough to be loaded.

I have lost one for sure, perhaps two.

I quit using Missile Turrets to watch the Bridge into Sanctuary, because I had to replace Three after Feral Dogs followed one Provisioner.

I see mine all the time on the road....I kit them out with combat rifles minimum and put them in full combat armor and some even have armored MM uniforms underneath. Have only lost one so far...and that was because she got replaced by a Synth.