It sounds more like deliberate head-rubbing which usually only occurs at times during the way but just watch her and get her to the vet and ask them to check her liver and for allergies if she keeps it up as you said in your OP she rubs it on just anything and you were worried about her getting a bald patch on her head which means she's pretty driven at the time by the pain or itch. A lot of dogs try to cover their food but that's usually unrelated to the other head-rubbing. True head-rubbing is painful to watch as you just know the dog is trying to stop whatever is bothering them and doesn't know what to do but they are trying to get it to stop! It's not just normal scratching or the occasional rubbing. That's when I have to step in and get them to the vet and checked out for what is causing the thing driving them to do it at given times. I tried to call Tibbe's nothing at first because I was so worried something really wrong was causing it so I understand how you feel - you want them not to be sick or bothered by anything. And he only did his at times but now, thank goodness, he's stopped it altogether. Whew!
Here's some links from Googling "head rubbing" and liver shunt. You can watch the videos and read some of the articles to find out more to compare those dogs with your dog's behavior.
https://www.google.com/search?newwin...33.SdlUuNFK-7E
What is dry sink? Not heard that term before.