What is hiding in our heap of sand?

The sand heap has become somewhat of a fixed feature in our garden right now. Landscaping is a lot of work!!!

So it should not be a surprise that something has taken up residence in it. I wish though, that I knew what it was. It only seem to be in there during the night, because when we disturb the sand during the day, nothing comes out. If we close the holes in the day, they are back in the morning...

I guess definitely a sign we need to get going...