Whose is it?

Here's a conundrum for you - you put something in your kitchen waste bin - does it still belong to you? You then empty the contents of your kitchen bin into your wheelie-bin - does it still belong to you? You put your wheelie bin out on the pavement for collection - does it still belong to you?  Your bin is emptied onto the cart - does it still belong to you?  I think most people would agree that once it goes into land-fill, it is no longer yours, but just when does it cease to be yours?  This is not an entirely pointless question - more and more people are it seems rifling through supermarket waste bins looking for stuff which is still edible or otherwise usable, and the question has arisen - is this theft?  Clearly this is stuff which the supermarket has no further use for, but is it still "theirs"?  As far as I am aware there is no settled law on the subject, but what do you think?