I'll give you an empirical guess based on nothing more than being a frequent pax :
Long haul affected much more than short haul.
Departing from base affected more than returning to base (makes me think that connecting pax are the issue).
Experience with BA long haul departing Heathrow is that easily 1 in 3 departures it is mentioned by the skipper as a reason for departure delay.
This takes no account of bags pulled before time, times when it is not mentioned, etc.
If you are looking at rates you need to consider both rate per pax and rate per departure. If BA long haul no-show rate was 1:600 and typical load was 300 pax, then it's a 1:2 chance of having to pull at least one bag (someone statistical may correct me on the calculation basis). I get the feeling its worse than that.
It's like assessing the rate for needing to do medical diversions - the larger the aircraft the more likely it is to happen.