So either someone in the motorcade fired a shot (which no one in Dealey Plaza saw) or someone was illegally burning something. Possibly a homeless person burning something trying to keep warm. There were homeless people in the vicinity - the three tramps.
None of the witnesses in Dealey Plaza saw SA Lem Johns jump out of the LBJ SS Follow Up Car and run up Elm St toward the JFK Limo either. Yet Johns and the SS claim he did this. There are numerous things that Did happen that "no one in Dealey Plaza saw". Do NOT permit that to shape your opinion(s). Also, anyone that has smelled "burning trash" and the odor of "gun powder" would Ever confuse the 2. Especially a DPD Policeman/Earle Brown.