WEe don't "sit" on hardly any packages in our hubs. We miss 5-10 a day (out of about 100k per sort) off each shift for a variety of reasons. There are feeders that are unloaded on the sunrise sort ,destined to our preload, that have a lot of what you would call "cold" packages in them. Anotherwords they aren't scheduled to be delivered until the following day because they are for extended towns or other locations that are O.K. to send to our building but aren't due until the next day for delivery. The boxes for the metro areas only are sorted out, and the other parcels are put into a "hold " trailer to be run later in the day by the Twilight shift. Those packages aren't missed or "left in building" packages because they are just not due for service as the others in the feeder going to our metro centers are. This allows UPS to make next day service to large areas including parts of surrounding states by ground but mixed in with the hot packages are other parcels simply so the trailers are sent out as full as possible. It's more much trouble for the hub to sort through and pluck out just the preload packages as late as 6:30 or 7:00 AM but it saves money for feeders to send out fuller boxes rather than hauling air. That is probably a "hold" trailer that you are concerned about. UPS doesn't deliberately sit on hardly any substantial amount of pieces. There are always exceptions of course, but it would be an integrity issue that would be suficient for a management person to lose their job to leave parcels deliberately sitting especially in the numbers that you describe, We have a lot of different flows, and split flows, to our Hubs nowadays in order to speed up our time in transit tto keep them shorter than anyone else. We still have the fastest time in transit for ground packages overall, amplified due to our recent changes, than any of our competitors. And it's very hard for FedEx, in particular, to meet the new transit times both because of their split network between air and ground and because their old trick of using excess capacity in their air network is becoming a thing of the past as they have less and less extra space available due to leasing their excess to the USPS.