For me scheduled scans seems to occur based on the following precedence:

  1. Current system status (on/off)
  2. System load at the time the scheduled scan meant to occur.
  3. Time of scheduled scan.

Therefore, if system load is above a certain system load limit, the scan is postponed (perhaps for a fixed time duration and also the system load at that next timing).

Note:If a scan fails to occur within a particular day, the scan is skipped altogether till the next scheduled scan (this at least is the way it happens for me consistently).

I think that a good feature to add would be an optional popup which is unchecked by default, to notify more advanced users who want to know that Avast! missed the previous scheduled scan. :wink:

P.S. I am not too sure what happens if the system is off when a scheduled scan is meant to occur.