Hi,
Maybe one of the experts here will correct me, but from the little that's in my own 4-home logs, I get the impression the logs are primarily intended to record problems avast has encountered within itself, or with the system, as opposed to suspicious things that it's turned up in scans.
If you want to see what's been happening durings scans (both resident and on-request), check the Data/Reports folder under the avast folder. There should be some TXT files there (if you have reporting turned on) which will give you useful histories.
Best,
Mike