That seems to happen alot. In fact there was an item about in I think Jeremy Vine on Radio two a week or so agin and it was mentioned again on Simon Mayo.
Some of the examples given were at a park or wood land. The dog did it's business and that was bagged, but as the owners did not want to walk around the park/woods with a full pooh bag, they hang it on a tree to collect and take home afterwards, the problem was that not everyone then returned and collected their bags. I have seen these in many locations, lovely trees, decorated with pooh bags, yuk, not nice anywhere and surprising in that part of London.
I also saw a large man let a dog drop one on a beach last week, he seemed not to have a bag with him and just scuffed a bit of sand on it. he could have at least buried it deeper.