Basically in event mode, record a few frames between events. Maybe 1 frame every 5 or 10 seconds.
Maybe this can be combined with the other idea about detection during continuous.
This is becoming more important to my use-case every day. I don't want to waste the disk space to record full-rate all of the time which would cause me to lose old recordings sooner.
Other systems I have used provide for continuous low rate recording (ie one frame every two seconds) then ramp to full rate recording whenever an event is detected.