What could be causing this leak near my chimney?
Without pictures of the roof, and the problem, this is very hard to diagnose accurately but here are a few common things it could be. 

1) The Roof could be leaking from somewhere near-by.  A rusty nail head could pop through a shingle etc..  You mention the roof is fairly new, this is unlikely.

2) The leak could be from in side the chimney if it isnt capped.. have you had the inside inspected / do you burn the fireplace?

3) There could be structural damage to the fireplace. Ridge cap could be installed in correctly, there could be a problem with the fascia boards.    

I live in the pacific northwest, and have never once heard of someone having to seal their brick fireplace. While its possible, I belive this to also be a waste of your time.  Unless this is a unicorn problem, I belive this problem could be solved with finding the leak, but I would need pictures, or to see it in person to make a better judgement.