A momentary glance, if that, it was really just a flicker in the corner of my eye. But I saw.
Yes, but what did you see?
There are so many ways this could go. Is it something scary or frightening, which could bring about the end of things as we know them? Or is it something positive, a smile on someone’s face as they catch a glimpse of you? Of course, that doesn’t need to be a positive thing, they might be an assassin sent to locate and eliminate you.
This fiction prompt can lead you down numerous paths, full of twists and turns. The skill is to bring suspense to what you write. How long can you build the tension before revealing what is going on?
No rules, just enjoy. Build tension and hold it for as long as possible, it is a valuable writing skill.