HK's

Where do these people come from? There were no cxn's or stby's left over from the previous day. The flight looks good with only UP's on the airport stby list, then in the wee hours of the morning, they show up? Can't be pax just buying a ticket, they'd be paying through the nose. And sometimes I see when the flight closes, the airport stby list shows say 5 coach open, but the flight list shows [(-3)(0)(50)] . I guess that never updates after the flight closes out. :huh:

Often, they are just unassigned seats. My husband had to call DL once to get pos space after 3 commuter attempts, so they put him as HK.
Spongebue can probably shed some light on this, but I think he's pretty busy these days. (or MrsDonna?)

Most airlines have a same day flight change option and, in my experience, a lot of people take advantage of it. This means that on the day of departure the flight loads are going to be very fluid until check in is cut off. My daughter-in-law got stuck in that just yesterday and spent the day at the airport trying to get home while I watched the flight loads shift and change on every flight.

Again, I'm not sure of all airlines' policies on the same day changes (some allow online changes, some require it to be at the airport) but the ones like Alaska who allow the passenger to change their own ticket to another flight that day generally allow the process to start at midnight. This would explain why you see a vast difference in loads during the middle of the night. Throw in the elite factor (they normally get a much bigger window to swap flights than the general public) and you have no way of knowing how something is going to look until you get really close to departure.

There are a lot of seats with Delta that you either have to be an elite passenger to select in advance, or you have to pay additional to select in advance, because they are considered "premium seats" (Any aisle or window in the forward half of the cabin.) Likewise, they hold a certain number of seats for assignment at the gate.
Basically, what this means is that if a flight is anywhere near full, there are always going to be a number of people with confirmed tickets on the flight, yet who do not yet have a seat assignment. Those are the HK's you are seeing.
The reason you don't see them until a few hours before the flight is because until they actually check in, they will not show up on the ASL (Airport standby list)
Always, always, always go by the "flight list" you speak of. The standby list that shows 5 seats open, doesn't actually show 5 seats open. It just means there are 5 seats unassigned, although they have been purchased by a confirmed passenger.

Another thing since nonrev's can print the seat request card 24hrs before departure, so you could see a lot of NR's verified and make you think there are too many ahead of you (AirportStbyList) and you won't get a chance if it's a full load. Then these individuals don't show up. I've had that happen to me before and still got a seat on a tight flt. It use to be that if you saw them on the list you knew they were "in" the terminal. A little more accurate accounting on the stby list.