Yeah, they were obviously following them but nothing suggests they would do anything but wait in ambush like the one at the college campus.
Burned Ones seem incapable of noticing people who are inside in some fashion. That's why they completely ignore the car, and they never bother people inside a building. They either hunt through scent, or they can only tell where things are if they are touching the fog.
Given that they started tracking them when previously there's been no indication they do anything like that suggests they have evolved some, or at least along a different path than others. We have no idea what all any evolution might entail. True, the MC could still anticipate their attacks, so we know that those had not developed their attacking abilities any further or differently. But we absolutely cannot say they would just wait in ambush like before. It's a decently likely guess, but we can't know for sure.
All of the above notwithstanding, the entire point was that they had developed further or different skills. We now know they can apparently track, but that's all. Your mention of scent and/or something tied to the fog (similar to a reverberation maybe?) would ofc explain why they don't seem to notice anyone inside buildings. Although the MC and Kallie were careful to try to hide at the diner, it's entirely possible it would never have detected them because they were indoors. In any case, they don't seem to wander inside even if there are people there unless there's some type of breach (like a broken window or open door).
I might be forgetting an instance where they went inside anyway, but I don't think so. The Office BO obviously was a special case. It might have been someone trapped inside and exposed to the fog there for too long, but it also might have been someone already infected enough that the process of changing was too far along. In the latter case, they might have gone to the office to accept their fate and just shut themselves inside to avoid being a danger to others. It's hard to say how that one ended up there originally, ofc.
Anyway, previously it seemed that people had to be practically right next to them (Kallie, College Kid, etc.) before they could notice them and attack. Around the college, none of them actually seemed to track. Noise, ofc, is almost certain to attract them. But they seemed more like trapdoor spiders or scorpion-tailed snakes up until recently. The line of them suggests either (1) Burned Ones develop different skills and so behave differently (the college BOs vs. the neighborhood BOs); or (2) they have evolved a little, adding actual tracking to their skillset. I think both are equally plausible.
