It's either stress or boredom or a combination of both. How long is he left alone for and what exercise does he get beforehand? Does he pant a lot or whine/bark just before you go out? It doesn't sound like full on separation anxiety but you could try making sure he is thoroughly exercised before you leave and also mentally tired by doing a short training/play session or even just playing hide and seek with his food. Or how about not giving him access to the front room?