A question about the prologue of part 2

We know that hordes pass through Jackson every year during the winter, as a migration, as Tommy explains. But why is this event not recorded in either of the two patrol logs we have access to, namely the route from the creek to Joel and Tommy's cabin? The records we find written are from a period of 1 year or almost 1 year in the record books. We know that Jackson patrols once a month. The patrols are recorded a month apart, as 2/10 and 3/10. Some logs say they found a few infected, but most of them say everything is clean. Could a horde pass through during this gap between one log day and the next, and the patrollers just kill the stragglers left behind by the horde? Could the hordes have been spotted by other patrollers than the ones we have access to in the log books? When Tommy and Joel go out to scout for reports of infected in the area, they are unaware that they will encounter a horde, and Tommy even mentions that they should return with a kill team. Or could it be that the horde that Tommy and Joel encounter are just stragglers from an even larger horde that passed by?