Albumin is an important protein within the body. It can be low because the body is losing it through the kidneys when they are diseased or through the intestines if diseased. It can also be low because the liver is diseased and not producing albumin. It sounds like your vet has ruled out liver and kidney disease with blood work and a urinalysis which leaves loss through the intestines (Protein losing enteropathy -PLE).
PLE can be caused by inflammation, infections, cancer, addison's disease, lymphngiectasia, intussusceptions to name a few. Clinical signs include weight loss, vomiting and/or diarrhea, and fluid accumulation. If your dog is having gastrointestinal issues it supports a diagnosis of PLE. Sometimes an intestinal biopsy is needed to determine the exact cause of PLE. Below are some links you might find helpful.
Please let me know if you have additional questions.