What's at the bottom of your garden?

There's something that sets our house guest barking at this time of the night.  She prowls around the garden, looking up, down sideways etc., but there is never anything disturbed by her nosiness.  Whatever it is, I wish it would go away. 
"Yesterday, upon the stair,
I met a man who wasn't there.
He wasn't there again today.
O how I wish he'd go away!"
W H Mearns