I have never once found a lost slide. But according to this site:
Every year, untold numbers of slides are dumped on the street. Some are thrown away by uncaring stock houses, some are discarded after a new digital camera is bought, some are just lost in the shuffle during cleaning. Only with your help can they be reunited with their rightful owners. So please, if you know of someone who has lost a slide, we hope that you will tell them about us and our cause. You can make a difference.