We have a company that we use for employee background checks when they are hired. My question is, is there anything preventing us from using that service to run a background check on a customer that is nowhere to be found? Our contract with the company allows us to use it for that reason. When employees are hired they sign a form to give permission for the background check, we don't have customers sign anything like that because it's not something we normally do.

Skip tracing on past due customers is a pretty common practice. You would not need written permission as long as these are not investigative consumer reports.