I just think all the art is a bit disturbing. What seems most odd is that there is so much of it about and yet when this was the Dalish homeland they never picked up on it? Just who did they think the images referred to? Then again, look at the Arbor Wilds. It was in the south of the Dalish homeland and yet during the 200 odd years they were there did no one attempt to approach it? If they did and were attacked by odd looking elven like people, did no one question why this might be? Given the modern Dalish are constantly sending people into ruins to try and recover artifacts, it seems odd that they didn't discover more when actually living there permanently.
I think you underestimate how BIG Thedas is. Dales were a vast land and at the time there were less people in Thedas than there is now. Then there's the fact that a relatively big chunk of history o elven Dales coincides with 2nd Blight and we have accounts of blight-related darkspawn activity in Ferelden with main darkspawn force active in the east. This pretty much implies that while they never helped humans, Dalish had their own war with Darkspawn, not on the same scale maybe, but that definitely seems to be enough to discourage any archeological attempts at searching through the pristine forests of the far South.
Basically, they had those big forests where nobody really lived and rare travelers tended to be lost without a trace - and then there appeared quite a lot darkspawn there. I find it perfectly logical that some ruins still remain untouched from the times of Arlathan itself. After all, these are just tiny dots you need to stumble upon in a vast forest that takes months to traverse... If it would be additionally concealed from most common scrying magic - finding by chance something like ToM is a nigh-miracle.