Had this in my old flat in Norwich, cold spot that genuinely seemed to drift from the hallway into the living room over about 20 minutes, no windows open, no drafts I could find. Grabbed my phone to take some photos obviously but nothing showed up which was a bit annoying.
Probably just air circulation doing something weird if I'm honest but it was strange enough that I still think about it now. Anyone actually managed to track one with a proper thermometer rather than just their hand? Feels like that would atleast give you something solid to work with rather than "yeah it felt chilly mate."