Is DBXL indicating the file is in use or is it SharePoint? DBXL maintains its own locking mechanism separate from SharePoint. The InfoPath client will lock the form in DBXL and update the lock periodically while the form is open. There is a potential that the form is not getting unlocked by the InfoPath client.
Run the following query on the QdabraDBXL database to see the status of the docid in question:
USE QdabraDBXL
GO
SELECT DocID, LockedBy, LockExpires FROM Document WHERE DocID = youdocidhere
GO