I found it! Many years ago (20-yes it's taken me that long) the memorial was moved from Old Town to Balboa Park. As I usually only went to the San Diego Zoo when I ventured to Balboa Park I never knew where it was located. In the past years we started using the Park more with home schooling field trips and high school graduations from River Valley, While we were down there I always wondered where the memorial was, but never knew.
On October 1 we went to a wedding and as we were walking away from the wedding-there it was! It was hidden behind a building. No wonder why we never saw it when we drove by scanning the lawns for anything that would resemble a memorial.
Here are the pics I took with my iPhone which has miserable focus.