Locating the Source of a Bad Smell in the House?

I have a bad smell in my house that comes and goes. I can't figure out where it is coming from. I had the fire department check the house and it wasn't a gas leak. It smells like rotten eggs or something sour. It only smells in the entrance to the basement.

We cleaned the shelves and trashed all that we weren't going to use anymore, but the smell is still there. There are no pipes of any kind in that area so I don't know what this can be. If it was a dead animal I'm assuming it would smell all the time, but it doesn't. Sometimes it doesn't smell and sometimes the odor is very strong. What can it be? Any ideas of what to do or what could it be will be appreciated.