I have a pet. I am responsible and pick up after it. I have been to sites where someone else left poo behind. I just pick it up and don't worry about it.
I try to look around the site the very last thing I do before I leave. That being said I suppose it is possible I may have missed some at some time in my 36 years of camping. Some of the parks I go to do not mow the grass all that often and it can hide in the tall grass sometimes. ( I don't camp at places that have the grass manicured like a golf course. What's the fun it that ? )