I discovered it similarly in a camping trip when I was a kid. We were walking around in near pitch darkness, and I saw out of the corner of my eye someone walk away from the group and try to hide behind a table or something. When I turned to look I couldn't see anything at all, but if I averted my eyes I could clearly see there was a person moving around crouched behind a table. I can't remember exactly, but I think I wasn't so much seeing them, but rather their movement, and mentally perceiving their silhouette.
I guess it looks sort of like this: https://www.youtube.com/watch?v=M83sF7_fYdM