That's the problem as I said avast is a resident on-access scanner - it doesn't do its own thing.
I suspect that the point you mentioned is a strong contender:
I have a backup program (Create Synchronicity) that is scheduled to copy files from one HDD to another. The backups are scheduled for later in the day (9-12pm) and there is a 'catch-up' backup set if the scheduled backup is missed.
Whilst you have disabled 'Scan documents when opening' - you need to read the rest of the notes on that option as it relates to MS Office documents and all other OLE files. So this would still result in non MS Documents being scanned.
As an avast user not an avast team member, I'm at a loss as to what to suggest as there is no setting to switch off something that isn't/doesn't have an option in the UI.
Unfortunately I don't see any of this on my XP Pro or Win7 systems - I have pretty much the avast default scan settings - windows indexing is off on both systems. I also have Everything on my win7 system, but it is on default settings. Just found it is an old version and updating now to see if anything changes.