“Had broken away” is correct and just means you don’t know what happened. “Had been broken away” is also correct, but implies someone did it - but it’s not said who. Maybe there’s some evidence of it, or it isn’t possible for it to have happened without intervention. It seems odd with a balloon - but for example - “the branch had been broken off the tree” - implies someone did it, you can see evidence that it didn’t just happen. So they are both correct depending on context.