Cankers (wounds) can occur underneath the outer bark from various causes, and then sap stains show up on the outside. Looks like a small area, so shouldn't be a problem unless it enlarges.

Most likely a canker at work. But I wouldn't completely rule out borer. They can cause sap staining like that without frass. I'm not as optimistic as beng is about the tree's long-term health, especially if it is canker. You could seek out an arborist for an inspection.