There is a damp patch in the corner of my garden cupboard, amongst the various garden instruments: a shovel and some tomato poles. It has been there for the last couple of months, and it grows and shrinks.
During the night it spreads and in the morning it fills the corner of the cupboard. I never see it moving. It waits until I am not looking to change its shape and dimension. During the afternoon it remains. It has also crept up the wall, and there it never changes, it stays there watching to see what I might do.
There are no insects to feed on its shadow, they leave it alone. I think they avoid it, being uncertain where it comes from. During the day I leave the door of the cupboard open, hoping that it might shrink, might even go away, but it remains where it is, perhaps diminishing in size only to grow again when it gets dark.
One day it might engulf the whole cupboard, leave just a very large growing damp patch. I think it originates from somewhere below, seeping slowly up to the surface.