The position's we're in are shut down on negotiations

That's a negotiating tactic. It worked.

You have three options:

1) Do nothing. Sounds like you are going to do this, but it's the worst thing you can do.

2) Bring it up with your boss sooner than later. Arrange a meeting with him (don't wait for a review, implies no importance). Your agenda is simple: a) You accidentally saw new hire's pay (don't get into a discussion about them). b) I'm being paid X but the market rate is Y. c) I work hard, meet deadlines, got promoted, etc...I would like a pay review to be a more realistic value of my level. Do not threaten anything, like leaving. d) They will ask what you want. Have two numbers. One you tell them, the one you are asking for, and another that is the minimum you will accept. e) They'll think about it and come back with an answer. i.e. either no, something between your current and your minimum, or between your minimum and asking. You now have a decision. Unless it was between your min. and asking, I suggest you move onto (3).

3) Keep quiet, don't quit, but start looking for a new job. You are more likely to get quickly raises this way as you can often demand more as you enter (as your new hire did).