This is a green bag. A green bag of food waste.
The green bag contains things like potato peelings and leftover scraps: the sort of things that can’t go on the compost heap. The green bag normally lives in a container. When the bag is full, it’s taken out to the green bin. Here, someone has taken it from the container and left it on the kitchen counter.
This is very much the COP26 solution to the problem of keeping the house tidy. It has the appearance of doing something to help the housing environment, but it achieves nothing. It’s actually offloading the real problem (taking the green bag out to the green bin) onto someone else, all the while giving the culprit the chance to boast about their green bag credentials.
Who was it? Investigations are ongoing, but I’m pretty confident it will turn out to be the dog. The dog seems to be behind most of the crimes committed in our house, including drinking the last of the milk, not putting the butter back in the fridge and failing to turn on the dishwasher.