Probably not often, but from the same day, I think chances are you'd remember. I heard they were missing around 5:00 pm. Had I been in the area that day, it probably would have registered. When I ride on the bike trail, I would remember if I passed two girls on bikes, a jogger, a person walking the dog..... not that I would remember faces and be able to give a description., but I could tell you if i saw two girls on bikes within 12 to 24 hours or so after.
BBM - I find it touching that, aft all this time, you still remember what time of day it was when you first heard that two little girls were missing. <sigh>