I have hit command-0 to fit to screen and you can see that somehow it is part of the canvas and will not let me expand any further.
If you mean it does not get any bigger if you increase the magnification, it probably isn’t part of the image, instead it’s probably a user interface indicator of some kind, such as a handle or object marker.
Those types of indicators are listed under View > Show. Try toggling the command View > Extras, which toggles the view of all indicators. If the unknown box disappears when you toggle View > Extras, then it’s one of the extra indicators. You can then go down the list on View > Show and toggle each until you find out which one makes the mystery item go away.
For example, if it’s a handle for a single path, it would disappear if you toggle View > Show > Target Path. Why do I use that as an example? Because I was able to create something that looks similar by clicking a single point with the Pen tool, and it becomes hidden if I disable View > Show > Target Path. (It also becomes hidden if I deselect Work Path in the Paths panel.)
When you are able to narrow down what it is, you can then delete it if you don’t want it. In my example I would use the Path Selection or Direct Selection tool to select and then delete the stray path point. But yours might turn out to be something else.