Being in a brick wall, I'd say yeah, it's likely been there all along and was maybe a small inswing type of window. All the better for your purpose, Robert; that means you've got a good jamb assembly that was already built with drainage, etc. (Sometimes when someone cuts a hole in a wall for a window A/C, they haven't done so in a way that's very weathertight.)
It probably will make your job of trimming in a fixed panel even easier. Good luck!