Not so much a bug as an irritation. To advance the Headless Gunman quest I need a sensor module, but they are simply not spawning anywhere. I've checked all the vendors multiple times over several in-game days (Diamond City, Trudy, Carla, Good Neighbor) plus random traders on the road, and gone to the sites recommended on the wiki, but have had no luck. I've also checked the bench inventories at my settlements in hopes that I had picked one up earlier and hadn't used it yet - no luck. I started this quest after completing the Molecular Level, which is the only quest that also requires it. So I'm not sure if I'm just having tremendously bad luck, or if it stops spawning after that. (on the other hand, there is no shortage of high-powered magnets to be bought - practically every junk vendor has one of those!) If I don't get one in the next little bit of questing, I'll just spawn one in using console commands, but this could be a frustrating problem for someone on Xbox.