Hello,
what type of Office clients are you using? Because the different Office versions support / use different types of locks - Office 2010 especially uses a lot of shared WebDAV locking, which doesn't show up in the Alfresco UI like the exclusive locks of Office 2007 (and Office 2003 on Windows 7) used to.
Depending on how Office has set the lock, they may be automatically released. But if Office set the lock without an expiry option, then it may remain indefinately. For an on-premise installation of a customer, we have implemented a small JavaScript that resets the relevant node properties, so an administrator can reset the lock.
Regards
Axel