We just ran into this above today. Single user (checked the lock files, yes all from the user's arcmap process), select the GDB in the ArcMap catalog window error message pops up that schema locks exist.
Here's a comment from a 2008 forum thread by Tom Sethre that I thought was helpful:
While exchanging emails with Mark S. with ESRI Support three weeks ago, I stepped through my test sequence once more. I opened ArcCatalog and tried to create a File Geodb on the network drive (in the same folder as my Personal geodb). It failed with a schema lock. Clearly nobody else on the network could have caused the schema lockout.
THE CRAZY-MAKING THING: I then closed ArcCatalog, deleted the New File Geodatabase folder, re-opened ArcCatalog and was able to create a new File Geodatabase on the second try. An attempt to copy a shape from the Personal Geodb to the new network File Geodb failed with the schema lock error.
MY THEORY
ArcCatalog is not allowing time for its lock release to propagate through the network. It looks to me like a timing issue.
I believe that ArcCatalog releases a File Geodb schema lock by deleting the lock file. Sometimes it may take a few milliseconds before the network drive can complete the file delete request and re-broadcast its revised directory contents. This would be consistent with the intermittent nature of this problem.
We found another cause to this issue last month:
If you use Catalog connections to the same folder mapped two different ways (ie UNC and letter drive) within your ArcGIS Desktop session, you can run into nasty access issues.
Don't do that!