Did they see a getaway car speeding away? The grassy knoll is not a secluded location. If you were on the sidewalk watching the motorcade, the grassy knoll is immediately upslope behind you. You could be at the top in five seconds.
Behind the grassy knoll was a train yard. They searched the cars and found 6 men. 3 turned out to be Watergate burglers. All had on cheap dirty clothes but shined military shoes.