Projects affected by deleting images in Organizer

I get this message sometimes when I delete a picture in Organizer. ==>"one or more of the items you have selected to delete from the catalog might be used within one or more projects."<== I wonder if anyone knows how this is queried by the program.  I know the "Slide Show" feature in Organizer allows identification of all images in a slide show so I have deleted all my Slide Shows but I still get the message.  I have Premiere Elements and I can live with the risk of deleting images associted with those video projects because it will identify a specific file that it can't find.   Any input would be appreciated.