No doubt this happens, it must be so rare for someone to put out poisoned half-eaten sausage rolls with the aim of killing a random dog that finds it, I think this is perhaps a teensy bit paranoid.
Bread rolls with bits of glass or metal in them, found in parks or along other pedestrian paths. They caught one woman doing it, but it is believed the news might have inspired other nutjobs.
But this isn't even about this particular half eaten sausage. A dog doesn't know the difference between a pristine sausage and a half eaten one, to them it's just meat. And the point is that dogs should be taught to never eat any meat on the street - because they can't think "uh, looks half eaten, probably fine".