Spent way too long last winter chasing warm lines along my roofline before I figured out what was acutally going on, so here's what I wish someone had told me early. When you see a warm band where the roof meets the wall, the first thing to check is wether it's a steady pattern or something that moves with conditions. Shining surfaces, expecially metal flashing and painted trim, can pick up reflected heat from nearby sources and look warm when they're not. Point the camera at a low angle and the reflection shifts, point it straight on and real heat stays put.
The other big one is sky radiation. On a clear cold night, surfaces facing up actually read colder than the air around them, wich can make the eaves look relatively warm by comparison even if nothing's wrong. So how do you tell? Go inside and scan the same spot from the ceiling and top of the wall. If the cold outdoor pattern shows up as a matching warm or cold streak indoors, you've likely got a geniune gap in insulation, ussually right at the junction where framing meets the attic boundary. If the interior looks even, it's probably just an exterior artifact.
If it turns out real, the usual fixes are topping up attic insulation right out to the edge over the top plate, or fitting baffles so the loft insulation doesn't get pushed back from the eaves by ventilation. Honestly though, a faint warm line at the eaves is one of the most common things you'll ever see, and often not worth losing sleep over unless the indoor reading confirms it.