Professional Treasure-seekers?

[Sardy]

""Professional stealth?" cried Balin, taking up my words rather differently than I had meant them. "Do you mean a trained treasure-seeker? Can they still be found?"" --from Unfinished Tales, The Quest of Erebor

I just re-read this passage and it struck me as quite intriguing. Just who were these "trained treasure-seekers" for whom Balin and the company of Dwarves seem to have so much respect, awe and admiration for? What great adventures did they partake in that they could have so impressed the unimpressionable Dwarves? And even more importantly, what has happened to them that they can no longer be found, indeed, the profession no longer seems to even exist?

One excerpt that really struck me was this:

""Ah! I see your drift at last," said Balin. "He is a thief, then? That is why you recommend him?""

This disparaging opinion of "thieves" on Balin's part strikes me in that there is a definite difference between the aforementioned "trained treasure-seekers" and a common, run-of-the-mill thief.

It would seem that the treasure-seekers were the stuff of legend... So who were they... any mention in Tolkien's works of them as a class or organized profession?