Ugh yes this is so real. Spent 4 weekends last winter chasing a cold spot in my bedroom and it turned out to be a gap behind the baseboard where the drywall didn't meet the floor. I had already added weatherstripping to two windows and insulated the whole rim joist area for nothing. A smoke pencil works but the cheap ones are kinda weak, and the IR thermometer only helps if there's a big temp difference. What finally saved me was turning on a box fan in the doorway and feeling around with a wet hand, the cold air hits your skin way faster than smoke moves. Still felt pretty dumb when I found it though.

What kind of shape are the mortar joints in before you even start brushing? I ask because with 1890s brick around here the lime mortar is usually so soft you can carve it with a fingernail, and even a poly whip at low speed will knock the face off it. Do you run the whip on a drill or by hand? On the really crumbly stuff I have had better luck doing most of the work by hand with a poly brush and only using the drill whip on the hard soot up top. Curious if you changed your speed too, because slowing down made more of a difference than switching from rods on the last soft flue I did.