I have a shower in the lower level (3-ft below grade) which I don't use and keep getting a noticeable sewer-like smell from the shower drain. When I have the smell, I look down the drain with a flashlight and 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?

If you are not going to use that shower try pouring some cooking oil into the trap, it won't evaporate like the water does.