Session: Error while trying to free lock for ci_session

I have reviewed the other few posts related to this error:

Session: Error while trying to free lock for ci_session

and cannot figure out the problem. Our site isn't even close to taking 5 seconds for page loads, in fact they are typically 500ms. Therefore, any other ideas on how to troubleshoot?

As a followup to this I configured memcached (the session driver we are using) to NOT use file locking. I confirmed this with a phpinfo page. Even after turning off file locking I still get this message.