I left mine at the side of the path on one walk (a long route in Gt Harwood with just one bin at the very start), with the intention of collecting it on the way back. It was in a bright pink nappy bag - couldn't miss it.
Someone else had tidied it up by the time I returned.
(When I say "mine" I mean "my dog's".)
Incidentally, I got some clear video on the dashcam of someone letting their dog take a dump while I was driving under the railway bridge on Blackburn Road.
Doesn't meet the criteria for getting a reward, though. The camera didn't show the owner walking away without scooping. I only saw that in my mirror as the lights had changed to green.