It is sad. I have done a lot of dog-sitting and they are constantly looking throuh the window wondering where their owner is.
Friends always leave some of the dog's favourite toys and treats and an item of clothing, and the dog's own bed and blankets.
Just give them plenty of cuddles and allow them on the sofa, maybe.
I have had 3 labradors on the sofa with me at one time. (mine and two from a friend )