Strange "injury?" on cat

Twice in the past year my cat, who is outside occasionally during the day, came in with a spot between his shoulder blades about the size of a quarter.  When it happens, all fur on the spot is competely gone.  There are no marks, no scratches, no bite marks, no ring, nothing.  Just competely white skin with no fur.  As the day progresses, the blood comes to the surface and it scabs over.  The first time this happened, it healed completely on its own, the fur grew back.  Now, approximately six months later, the same exact thing has happened.  Same location, no fur, as the day progresses blood comes to the surface and it starts to scab.  This time he was able to scratch it.  I took him to the vet who shaved the area and covered it with gauze to protect it.  The vet thought the skin looked dead due to its dark color.  The vet has no idea what happened.  There is no infection. No marks.  No scratches. Nothing. Anyhow, the initial scab has come off.  It is still healing and I have a follow-up with the vet this week to see if it is necrotic skin, needing sutures or what.  My main question (sorry this is so long!) is I am so nervous this could happen again to the poor little guy.  Does ANYONE have any idea how this could happen??  So strange.  No marks.  No infection.  Help?   Thanks.  

Do you use a flea spot treatment and is it in the same place where that is applied?

Same question my vet and his staff had. No flea treatment used. The vet also insisted the cat had scratched the area, removing the fur. Nope. Not a place that he scratches. One second he's normal.... he walks in the door and has a bare spot with no apparent cause. The first time I thought maybe he ran under a fence and scraped the hair off...but that doesn't make sense since there is not one scrape mark. Any other ideas? I've spent a fair amount of time trying to figure this out. Would hate to keep him indoors all of the time, since he loves it outside, but... this can't keep happening. (My vet also wondered about a spider bite... but same area twice? Plus, there's no mark... No infection either. Baffling!!)