DH has several entries for Mesh armour, but I'm left wondering if all Mesh is of xenos design (Xeno Mesh being pretty obvious). I know that old fluff described some Imperials as using Mesh (arbitrators wore it under their leather uniforms), and (I think) ogryn used to wear it, but I'm wondering what the current stand is on it.

I would say it's one of the technologies Imperium has copied from xenos, because it is mainly used by eldar but nevertheless has availability of rare while all other eldar technology (shuriken pistols and catapults, for example) have availability of very rare. Also, I don't think it would be available as starting equipment (for at least Arbitrators, can't remember if some other career got it too) if it wasn't made by humans.

On the other hand, actual xeno mesh will most likely draw attention of local Arbites and Ordo Xenos if openly worn.

As said above, Mesh armour was originally a xeno design (namely the Eldar) and it was either captured, stolen or bought by the Imperium and reverse-engineered.

Ogryrns and Commisars used to wear it back in the day.

My view is all Mesh armour in DH is man-made, except for Xeno mesh which is usually Eldar or Dark Eldar armour, and wearing it is likely to earn you some bad attention.

I always thought of Mesh armor as just being a catch all term for armor that wasn't rigid. I picture any of the billions of different technologies that different planets would have for making things like bulletproof chainmail and kevlar-esque fabrics.

This is, of course, not including Xeno-Mesh which would of course be a breed all it's own.

If I remember correctly, mesh armor traditionally filled the gap between flak armor and carapace armor. For instance, in Necromunda, flak armor would be a 6+ save, mesh 5+, and carapace 4+. The benefit to mesh was generally that it was lighter than carapace, and that was the trade off for the lower protection rating.

The explaination behind mesh used to be an armour constructed from tiny scales of thermoplas bonded together to produce a tightly-woven material similar to chainmail or reptile scales. When struck the material becomes rigid at the point of impact, spreading the trauma across a larger area. The thermoplas also disperses heat and energy rapidly, making it just as reliable against energy and incendiary weapons.