I noticed that none of the art in the Core Rulebook (or anything I found googling Deathwatch for that matter) shows company or squad designation markings on DW armour. Is this because of the special nature of DW's cross-chapter enlistment? Or is it just artists genercising their illustrations?

It's because Deathwatch Marines aren't separated into Companies or Squads really, and as such the shoulderpad that would have shown that info instead has the Deathwatch symbol on it. The images in the art are correct.

Good to know. I suspected it might be something along those lines.

actually the shoulder pad that would show the Chapter badge now shows the Deathwatch Icon and the chapter badge has been moved to the side which normally shows a squads designation (type)