Air leakage into interior walls including plumbing or exhaust flue penetration into attic above. Top plate air leakage present as well.
Large exhaust flue chase open to the attic above. Dropped Ceilings also look similar. Infrared Picture taken on cold day.
Air movement through swamp cooler and around air movement at top of wall.
Air leakage under exterior door. Install bulb type weatherstripping or change / raise threshold.