I don't think that behavior is unusual...our dog Rocky does it too. When Jeff leaves for work in the morning sometimes (not all the time) Rocky will howl for a minute or two...and I'm still here! I am usually sleeping, but that wakes me up. He probably doesn't realize that I'm still at home. He has also done it when we leave to go someplace together. A few times we had gotten into the car but had forgotten something in the house, and when we go back in to get it, Rocky has been howling.

Rocky has a companion (another dog and three cats), so lonliness shouldn't be an issue. Sometimes I think it is just his way of saying goodbye.