One good addon to use is 3D Print Toolbox, it comes with Blender and it enables mesh checks for topology errors.
When you see a loop stop the most common things to look for are inside faces, double vertices maybe, a rogue vertex on a face somewhere, a face that has been ripped, Ngons, triangles, so anything that is not a clean quad. And you don’t have to look far, only in the face where the loops made the stop, so it shouldn’t be hard to go luring and finding the culprit.
