Thank you for your replies everyone. Allow me to clarify further so you have a better understanding of the circumstances.
In June 2012, $29K was loaned from my ex's parents, paid onto our Home Equity Line of Credit.
The payments are alleged to have been in denominations of $500. Over the course the course of 48 months, there are approximately 16 months of missed payments.
The only evidence of the loan is my mother-in-law's chicken scratch on a loose piece of paper that could have easily been forged. But, this potentially classifies the loan as a 'payment on demand' loan, without interest.
I want access to the parents' records, so I can find all the 'missed' payments, which I'm certain to find.
My goal is to prove that the loan was paid down more than they admit - not that the loan never existed.