My girlfriend and i were sitting outside of ny workplace taking a break. we were watching the sky as we do frequently and discussing the vapor trail left by a passenger jet. at that moment we noticed another jet traveling w/nw across the sky. it was obvious as to the vapor trail being left behind. all of a sudden there was a small (visually) glimmer as if a very bright reflection in the sky. the reflection lasted only a second but it was enough to notice it came from a small round object just below cloud cover. judging by the distance of the object and the plane it followed it looked to be larger than a passenger plane. this object was following the exact path as the plane but was offset and trailing a slight distance behind it. it continued to follow the plane keeping almost an exact distance from and same path and speed as the plane until it was out of sight. being prior military and a frequent traveler i have seen many strange aircraft personally but nothing ever like this.