Nuns and sun

I love the view from my desk. It used to be a car park but now four benches sit in the shade of three large London Planes.

It's a place where not much is going on - a pause in a trip to the library/car park/college.

Today it was occupied by two nuns in biblical garb, eating burgers and chips, swinging their legs and laughing easily with three friends.

For ten minutes there was what looked like a friendly, engaging and good natured discussion. Lots of smiles.

Then they moved on to the car park.

What is the name of that sadness that follows people as they are leaving?