Canine Marking-Boxer

Question
My two year old boxer is a very well behaved animal.  He was trained with my metro police department for gaurd work, tracking and obedience.  His health is very good but he does do one very unusual thing.  When taken on long walks or to the dog park he often produces three to four successful fecal samples and then spends a significant amount of time during these jaunts trying to or secreting a clear mucus from his anus.  I've taken him to the vet and everything seems to check out fine.  Is he marking with feces or does he just enjoy what he's doing?  I just don't know what to make of it.

Answer
I doubt that he's doing it because he likes it.  In fact, it may have more to do with what food he is eating.  Some dogs produce better stools on certain foods than others.  For example, I have one dog that cannot eat Wellness.  That's a great food, but my particular dog gets runny poop from it.  She does very well on Innova.  
As to the number of times the dog defecates, that's an individual thing, too, and some dogs seem to need a walk to evacuate everything.  I've noticed that some of the Spaniel breeds like to walk AS they evacuate.  So, I guess the one thing you could do is to try a different food, and see if his stools become firm with no mucus.