From the way you described there being zero gravity drop and activity only after you opened the fermenter to check, I'd say to dump it and start over. Brett can be a slow acting yeast, taking months to mature, but by itself it does want to consume sugar.
I'm pretty confident that nothing got into your wort. However, once you opened it, all bets are off. I'm basing this on your report that gravity has not changed. You should be able to smell the Brett signature and see an appreciable drop in gravity (at least 3 to 4% worth). Even the smallest amount of fermentation (at even 0.5%) kills pathogens. With zero signs of fermentation this far along, I'd start over because of an abundance of caution.