I was exploring in the howling pass, and I just entered the caves. I had a solid wall on my Left, but then I noticed that the left arrow key was still blue. So of course, I clicked it. It got me to the small room, with 2 ancient treasure chests. Is this meant to be a secret passageway, or did krin just miss putting that little cave entrance on it? I just wanna know before I start wasting my time searching around for more of these awesome secret passageways.

Yes, those are meant to be there. In the Howling Pass, along with all the other festival maps, there are secret passages that lead to different places with secret treasures. Those can be found on Tori's maps. (The 2019 edition works)