It was always inevitable that Rise of the Tomb Raider would get a follow-up, and almost as inevitable that it'd be called "Shadow of the Tomb Raider" (games with "shadow" in the title are popular, you see). So it's hardly a surprise that a new leak can be added to those which seem to confirm that Shadow of the Tomb Raider is in development.
The first emerged in October, when someone noticed a fellow commuter flicking through Shadow of the Tomb Raider marketing guff on a laptop. Eight months later, and marketing guff has struck again, because this time a California-based company called Take Off appears to have jumped the gun uploading stuff onto their website, resulting in this being found:
The top line is a collection of logo options (note the image on the far right features art used to promote Rise Of..., in other words, it's placeholder). The bottom section shows a handful of key art proposals, illustrations which may serve as inspiration for the main marketing art for this third instalment.
The art has been removed from the website now, which only serves to raise my suspicions that the game definitely exists. I was surprised Shadow of the Tomb Raider didn't appear at E3 last week, but in any case, I think it's safe to say it'll come eventually.