Some mail is sent out metered if that day's volume from a print partner doesn't meet the minimum volume per mail type USPS requires. This means the mail pieces are not able to be batched with the other mail pieces and have to be mailed separately. This mail can potentially be mailed without a batch ID, making it unmeasurable in our system, even though it's en route to its destination.
Ideally, with metered mail, the rest of the tracking events should populate, but we've seen cases where the events didn't populate at all. Though we see scans for the majority of our mail, USPS does not guarantee scan events for every mail piece that goes through its sorting processes.