Thanks for the replies. I find that a dogs senses are much more acute than human ones. When in their prime anyway...older dogs almost invariably loose their hearing to some extent. The same lid worked with our other dog Monty, another pug like Rupi in the avatar but a fawn one. I did try the experiment with a different lid and most of the time, with both of them, no response.
Even when he is in the house, with the windows closed and with no view outside, the dog knows when other dogs are walking by on his patch. It's amazing.