Sounds like a challenging problem. Maybe an air quality sensor would help? I don't know if pot smoke would trigger one of those at the levels you are concerned about. The BME680 is supposed to be a good sensor for air quality. I haven't used it, and some others on this site have not been that impressed. See this thread: https://forum.mysensors.org/topic/12197/best-voc-sensor-for-detecting-a-wide-range-of-voc-s Maybe look at some law enforcement sites to see if there is anything they use (besides dogs : ).