Hi there. I invite you to go over to the ADHD forum and see what others have posted regarding this. I know people have posted in the past about fecal smearing. You may also want to post. Sandman has a lot of good information. Good luck.
My daughter is 9yrs old and has been doing the same thing since she was 1 1/2. It annoys me to no end and i can't seem to get answers! I'm at wits end with this the only thing she has been diagnosed with is ADHD. Please don't feel alone in this as I have for so many years. Good luck hope you find answers and if so please share! God bless
Page 415 of this book may help:
http://books.google.com/books?id=HnVOj5_rLpIC&lpg=PA415&ots=19tqZi77g2&dq=Prader-Willi%20syndrome%20feces%20smearing&pg=PA415#v=onepage&q&f=false