Warning related to parallel routes, but I'm not using that feature. | Next.js | Page 1

I'm getting a confusing warning at the moment.
I'm using the App router, and it comes up when returning a notFound() from a dynamic SSR rendered page, which I conditionally do if the id of the route exists in my db.
It does not happen when the page returns content as normal.

The warning is:

No default component was found for a parallel route rendered on this page. Falling back to nearest NotFound boundary.

Which is odd because I don't have any parallel routes in my application.

Code for the SSR page:

export default async function SubReckonPage({
  params,
}: {
  params: { subReckon: string };
}) {
  let subReckon = await api.subReckon.byId(params.subReckon);

  if (subReckon == null) {
    notFound();
  }

  return <div>{subReckon.name}</div>;
}

The warning doesn't seem to effect the rendering of the page at all, but it is annoying, and I'm wondering if I'm doing something wrong here that may cause issues in future.

Folder struture attached

#Warning related to parallel routes, but I'm not using that feature.