clam on "Living in row home-got BBs from neighbor (3 homes affected)"

So, after we found a few BBs in our house (now gone), we found out that a neighbor 2 doors down has an infestation in his house. We live in row homes and I believe we share the same plenum space.
I reached out to the landlord and he is taking care of this, however the guy who has an infestation is a dirt ball and there are no guarantees that this won't happen again. We are trying to figure out how exactly the BBs made it to the house next door. to my house and the next house over. The only plausible explanation is that they migrated through the plenum space. However, if the infested house is occupied and they have plenty of food, why are they migrating? Is it possible that the tenant in the infested house tried fumigating or bombing the place and the bugs scattered? We are trying to caulk all of the cracks in our house and seal ourselves off as much as possible. But to spread the infestation into 3 homes?? Any thoughts or suggestions would be much appreciated.