OMG.
I have this going on (and have, since about 2006) with a Red Sunset. I've had arborists out who didn't know, people here insisting it was lawn mower damage, you name it. But no one knows. Mine, too, is in the middle of a garden, so no mower.

It has spread to an English Hawthorne, a Sango Kaku, and on to an October Glory. Across the yard, a Trompenburg has started. So geographically, about 100 yards, and cross species.

The only helpful info is that yes, it does seem to be healing itself. The other suggestion was to spray the trunk with a 50/50 solution of bleach and water. If it is a fungus, which is what one of the arborists thought, the tree is fighting back.

The original tree has wood pecker holes in the bark, telling me it's fighting bugs. If I limb it up, it starts another ulcer-ish area like this. It's still growing, but I happen to think it might be stunted. The English Hawthorne finally broke in half in a wind storm, and the October Glory isn't deeming it serious enough to pay it any mind.

Truly, you have exactly the same (looking) thing I have going on. I have a cell camera now, which I didn't then. I'll get a pic and post it for you to compare. I'd be very interested to see what you come up with.

Edited to add:
I went back and got the thread for you and I see I never got to post the pictures then, either. The other trees mentioned seem to have shaken this off, because checking them, the scars are there, but they've grown out of it.

I promise pics.

Here is a link that might be useful: Original thread, not that it's any help..

This post was edited by CEFreeman on Thu, Sep 11, 14 at 8:55

West side damage = sunscald = typical of AB maples. Happens all the time to this cultivar. Your specimen has a very nice callus rolling over the dead wood. I'd leave this one be and see what happens. IMO it is likely it will heal over the wound in a couple more years and 10 years from now you would never know anything was wrong with the tree. You are lucky, more often than not, sunscald on AB maples makes them prone to breakage during high wind events like thunderstorms. Yours seems to have escaped this fate so far :)

You still need to confirm your rootflare is visible and healthy looking, though.

That might be the case for ABs, but how would one explain the Red Sunset, English Hawthorne, Sango Kaku, October Glory, and the others mentioned in my original post, way back when?

Here, finally, is my Red Sunset: