I have questions....

You're right. It does look like that shiny, smooth, flattened, twine-like, stuff that can be used to tie around packages.

 

Now, why it would be outside in the leaves and dirt down in the window-well...:shrug:

Maybe it just blew down there from outside. From a package delivery at another time? I doubt if it's got anything to do with the crime.

 

My thoughts exactly. There was lots of wind-blown debris in the window well.