Sometimes, people will leave bags in a prominent position to pick up later if they are near the start of an out-and-back walk. I don't do that myself but I can understand why someone might prefer to do so. It is therefore possible that the bags you see hanging around might belong to someone on a walk.
The pockets of every coat I own are filled with (empty) poo bags and (half chewed) tennis balls. It can be quite embarrassing when someone takes my coat in a restaurant.
When we go for a long walk I carry a rucksack containing a carrier bag. Poo bags are stuffed in there during the walk, then the carrier bag goes into a bin at the end. The only place this system fails is at Milford, where there are no bins. Under the cloak of forum anonymity I confess that I have...sometimes...deposited my carrier bag in one of the bins thoughtfully provided outside nearby businesses.
That's as specific as I'm getting.