dog has been vomiting for a week and cause is unknown

Question
My dog started showing signs that he wanted to throw up on Sunday last week.  He didn't throw up that day, but threw up the following day.  He's been throwing up around every six to eight hours since then, with a small amount of stool since he wasn't able to keep anything in.  I took him to the vet on Wednesday.  They did an x-ray, blood tests, potassium IV (to rule out an ileus), and kept him overnight for observation.  Results from the blood test were normal and the vet could not see anything in the x-ray although he did have some gas.  We got him back Friday afternoon because he seemed to eat a little bit at the vet Friday morning and didn't throw up.  When we brought him home, he vomited later that night.  He continued to vomit intermittently throughout the weekend.  It was mostly mucus.  He's been lethargic and has not had an appetite for dog food.  I cooked him fish and potatoes (the vet at first was thinking he had IBD)and he ate a little bit of that.  This past Sunday it seemed like he was getting better as he was throwing up only once each time (previously it was a primary effort with most of the mass thrown up at that time, then a follow up throwing up shortly after with mostly mucus), with hardly any food in his throw up, less mucus in his throw up, seemed a little more alert, and kind of showed more interest in food.  Since he's been home from the vet he's been pooping regularly, but with not as much mass and more moist.
However, since he was still throwing up this morning (no food in it though) I called the vet again.  We took him in for another examination.  They performed another x-ray, tested for parasites, and then we decided to do a barium test.  The first non barium x-ray did not show much different.  The gas in the intestines were in different areas.  The vet did say she wouldn't think much of it for a dog that had just eaten a meal, but for a dog that hasn't been eating that much his stomach looked big (his stomach looked about the same size in both x-rays taken thus far).  The parasite test was negative.  The barium test has not quite been completed, but after the first four hours, most of the barium was still in his stomach.  The vet commented again that his stomach seemed pretty distended.
I have not noticed him eat anything unusual and my dog doesn't usually eat random stuff, but of course you can't rule it out completely.  I will find out the results of the barium test tomorrow morning.  The vet said that if there is still barium in his stomach tomorrow morning, this is probably an indication that there is something in his stomach and exploratory surgery would be the next step.
My dog is 5 years old and has had surgery on his leg for a histiocytoma.  I regret acting so fast then to remove the histiocytoma as it turned out benign (I was alarmed by the angry appearance of it and what seemed like rapid growth, but in hindsight, it had started to shrink and dry up from the antibiotics he was taking before the surgery and I think it had only become infected from him licking it).  The surgery did cause him a lot of discomfort and pain during his recovery, he still has the scar, and I think it aged him a bit more.  I'm hesitant to put him through that again unless it's absolutely necessary.
Are there any other potential causes that have not been explored?

Answer
Jennifer,
An exploratory and a leg surgery are by far two different things. I wouldn't hesitate to have this done if the barium isn't traveling. Distended stomachs are not a good sign. However, the only alternative left to you before invasive surgery is an ultrasound.

That may or may not show something in there, and you will still have to go with the laperotomy, but you will be out another $200 or so.

I think you vet is right on track with this problem. Vomiting is usually an indication of an upper digestive track blockage but can be from a lower one too, in other words one lower down the intestines.

Go with her recommendations. He will have very little soreness afterward but most importantly, she will discover what is going on inside of him.

"I have not noticed him eat anything unusual and my dog doesn't usually eat random stuff, but of course you can't rule it out completely"

This is the absolute truth with dogs. Something as small as a golf ball can cause this but not be seen on an xray. I know, we have seen plenty of dogs present like this.

I hope he makes a complete recovery. Please let me know what you find out.