What's lurking at the bottom of my garden?

There are two compost heaps in our garden. One is an old-fashioned standard wire and wood surround whilst the other is a square plastic modern construction that is completely sealed with a flap-off lid. When I say completely sealed that does not account for the small slots just above its base.

A few weeks ago some type of creature began to enter the plastic compost bin through the small open slots. It tunnelled its way through the mounds of vegetable peelings and pulled some of them back through the slots. A mound of soft dark loamy earth appeared between the small garden shed and the compost bin.

So far, although I have poked about in the bin, looked in the space under the shed and generally glanced around and about I have not been able to discover what this little (it must be little to have squashed in through the small slots of the bin) creature is. The cat has taken a particular interest to this part of the garden and regularly does an early morning sprint to this spot. Yesterday she ran straight to the back of the shed where she patiently sat. After an hour or so she began to play "peepbo" with every member of the family that entered the garden ... but there has still been no sign of this mystery little creature.