There is the clue to how the fish got there; doesn't matter if it is wild or stocked ;-)
I have a stream that I fish that I discovered in 2012. It produced one my largest brookies to date , at 13.5" and in all the years I fished it, it was 100% brookies. About three years ago, someone sent me a link to a YT video, where I recognized the stream and where I also saw undisputed video evidence of browns. This stream does not flow into a stocked stream (at least one not stocked, except dozens and dozens of miles away, well upstream nearer to the larger stream headwaters, and where the water quality is only beginning to be degraded by agriculture). Made my own confirmation of browns in the stream a few years ago but am glad I got to experience it when it was 100% ST.
I hope I don't know the stream you caught your wild brown in, because I know a few class A streams that I would say are 100% wild brookies. And browns are here to stay but I really like fishing some of those last remnants of streams, where brookies make up all the population.