Is it really 'Finders Keepers'?

Returning stray shopping trolleys to the supermarket bay today, I found someone had accidentally left in one, a pack of two Custard Tarts Having missed my lunch break through an altercation with a customer/motorist which led to an interview with our manager, I am starving and as they sit here on my desk, I wonder just who has the moral high ground viz-a-viz ownership, me or the supermarket?