What does it mean when a DISA/Crimcheck background check comes back with an "inconclusive SSN trace"? 

According to DISA/Crimcheck, there are typically two reasons why the Social Security Trace comes back questionable:

1) The trace could come back to a completely different person.

2) The trace could come back without finding the individual at all if the candidate is young or relatively new to the country.

This happens because the candidate has yet to develop a credit and/or residential history of their own. To be safe, employers can always request a copy of the candidate's Social Security Card and contact the local Social Security Administration office. The SSA can verify the validity of the number matching the candidate since you are acting as the direct employer and have a right to verify this information. Unfortunately, DISA/Crimcheck cannot take this additional action because they are a third-party verifier.