My own theory at this point is that the fog itself is a sort of macro organism, operating as a hive mind and incorporating other living things into itself, mostly people but I suppose we might see some burned dogs or birds at some point. People in buildings are invisible, though the controlling entity may remember that people did go inside somewhere, and so can send witches to scratch at the doors and ask for help. The normal process is to leave people outside alone for the most part and let them expose themselves so it can work on them, maybe ambush them if an opportunity presents itself, but otherwise just be patient. Killing a witch changed this, and they got bumped up the threat ladder to being a Problem that needs to be addressed.
We also don't know what happened to the witch's body. I don't think Eve mentioned moving it. Maybe someone/something else picked it up, or maybe she got better. Heck, maybe burned ones that we've killed are not in the habit of staying dead in general.
Entirely possible that the process is ongoing, or that the regular rank and file burned ones are essentially the failures of the process. Is our large friend the result of a single person who got big, several regular sized people that fused together, or some other option?
I'm also assuming that the burned ones and other fog creatures basically need the fog to live much like we need air. Once you're part of the new ecosystem, the fog contains something vital for you. It's why the guy turning into a burned one in the office smashed the window, and it's why they don't come inside. Well, the big guy might smash some windows if they do know someone's in there to let the fog in.