Hi: I second shot a wedding reception last Saturday and my on-camera Canon Speedlite 600EX flash (on a Canon 5DM3) was triggering the lead photographer's two Godox off camera flashes (she had a Nikon camera -- not sure of the details of the lead's flash set-up).
The back-story: I had one off-camera flash and noticed I was triggering the lead's flashes but not the other way around. I tried various settings (ETTL, manual, optical, radio, different channels, etc.) just to experiment. Nothing worked. I finally took my off-camera flash down and off entirely.
My on-camera flash still triggered the lead's Godox system. I have no idea how this is possible because it was strictly on camera flash only in manual mode.
Does anyone ever experience this? What did I miss?