I do have a SharePoint (2013) site collection that is stuck in read only mode.

I'm unable to unlock from : 

 - Site Quotas and Lock in Central Administration (option is greyed-out)
 - Stsadm : operation completes successfully but no change
 - Using PowerShell : same behavior as stsadm

I reviewed the associated content database to ensure it was not in single user mode neither in read-only which was not the case.

Anyone knows how SharePoint is persisting that information in the content database so that I could have a sneak peek ? Any other way to unlock this state ?