SharePoint typically creates two lists when working with workflows. The built in workflows attempt to utilize a Tasks list and a Workflow history list. Over the past few days I have had an issue with an approval workflow rejecting the approval without any user interaction. In order to track down the problem I had to turn to the workflow history list. This list is located at SiteURL/lists/Workflow%20History/

In looking at this list (see screen shot below) I was able to track the workflow through the entire and could determine what actions were being taken by the user and by the system.