What Causes Fluid Leakage Out Of The Abdomen?

my husband had hernia surgery over a year ago, they placed a cow skin piece under the stomach muscle. He still leaks a yellow/ brown fluid out of a hole in his stomach. three months after the surgery when we noticed that his abdomen was still leaking fluid he went bck to the hospital. they were convinced that he had another hole in his intestines but couldn't find any. he does not feel sick in any way but he is still leaking fluid out of his abdomen.