I run into a same problem in Webi that if I stay in editing or viewing an instance for more than 10 minutes idle. This is a known issue and is addressed and fixed on FP 7.1 and above.
See SAP KB 2101612 - Temporary web intelligence reports created in ~WebIntelligence folder after the auto save time delay is elapsed when using Live Office on http://service.sap.com/sap/support/notes/2101612.
Regards,
Jin-Chong