HK's are usually either folks who missed their earlier flight, or were on an earlier flight that cancelled. The night before might tell you something if a flight canx and they rolled people over, but there can still be some show up a few hours before departure.

Example: That mid-morning flight looks good the night before, but if you check it after the early-am flight departs it is now full of HK's...all the pax who overslept or whatever and missed the early flight are now on your flight. Ooops.

If there is a really early flight before yours, always assume a few rollovers, especially if it's a big airplane.

If you are at the hub, expect more rollovers as the day progresses...they tend to accumulate. Last flight out of ATL on a busy day...fuhgedaboutit.