I made a brine in this saucepan earlier today, letting it cool within the pan for about 8 hours at room temperature. Is a non-stick pan considered "reactive"? Should I throw out the brine?
You are fine. Cast iron or "bare" aluminum you would not be.
Glass (pyrex) is the best for not transmitting any foreign flavors.
As long as there's no scratches in your non stick saucepan, its non reactive - so I've always thought!