thanks for the continuing discussion on this - it's a topic that is worth digging further into
I quite like the idea of 'semi automatic' detection (but not quite in the same way). The problem with turning up the sensitivity is false positives, but as noted above the eye is better at spotting than any algorithm (at least for now). Perhaps have a more sensitive detection mode where if anything shows up the frame is 'held' - not added to the stack, but not discarded - then later you can check and click yes/no for each held frame...
cheers,
Robin