Intermittent HTTP 500 error returned to client

Question

Client gets intermittent http 500 error when calling our WCF based rest service. When we check our IIS (IIS 7.5) logs there are no http 500 errors matching the reported errors. Nothing is trapped in the service error logs. We are at a loss where the http 500 errors are coming from. If the IIS logs do not have a record and no exceptions in the service error logs, where else could the http 500 be generated from? This is happening intermittently for them which makes it even harder to troubleshoot. Any suggestions for how to track these kind of errors or what might typically cause a http 500 to be returned when nothing is found in the IIS logs? IIS logs do show 500 errors just not for these specific reported ones. Any ideas or suggestions greatly appreciated!