I have a shower in the lower level (3-ft below grade) which I don't use and keep getting a noticible sewer-like smell from the shower drain. When I look down the drain with a flashlight I cannot see any water. If I run water into the drain the smell disappears for a time (couple weeks) but eventually comes back.

Why is this happening? Is this normal? What can I do to prevent the smell without having to remember to run water in the drain?