Mystery injuries scare me

So, Daisy went and got herself a boo boo today.
I dislike, very much, coming home to discover a bloody spot on my dog. It's a scrape kind of thing, about the size of a quarter, and it's almost perfectly round. It left blood on her fur, and will probably scab up into something dark and gross in the next day or two. I clipped and cleaned away the hair, cleansed the wound, and then put some Neosporin on it.
But then the fun began: the hunt for whatever in the hell did this to her. And no kidding, I could not discover it. She didn't get it behind any toilets or under any cabinets in the bathrooms, because she always leaves evidence when she's been dicking around in there. ... It wasn't the right kind of wound to be made from the new perimeter fencing in the backyard. That would have made more of a stabbing wound, or a long scratch. Also, there was no hair on the fencing anywhere. So, not there. ... It wasn't in any of the closets, or under any tables, or anywhere in the house which could have made such a mark. Brian contends that it was on the doggy door, with her jumping through it too quickly, and poorly judging the upper portion. I don't buy that. She's too little for that. Also, no blood or hair was found on the doggy door itself.
The mystery continues. Whatever it was, I hope she doesn't do it again. (And it's kinda gross, so I'm not going to post a picture of it.)