Put a bowl of vinegar in the closet overnight and see what happens. If the smell is one that is still being given off by whatever it is, the smell will come back when the vinegar's removed, otherwise it hopefully will go away.

Could you repaint the closet? When I had a mouse die in the wall, after I got him out I put a big basket of cedar chips in the closet, that took the odor out and made the closet smell really good. .Debbie

It doubt the walls and flooring have anything to do with the odor unless this is a new home/addition.
Is there a bathroom backing any of the walls of the closet?

If so, have a plumber come out and check the vents & sewer connections.

Here is an update. The closet is on an outside wall opposite our front door so there was no vent pipe issue. My husband painted it Saturday and the smell is almost gone. The linolium floor no longer seems to smell but the back of the closet door still stinks. It has a mirror on it so we did not paint it Saturday. I am going to take a MR. Clean eraser to the exposed paint and hope that finishes off the smell. If it doesnt we will have to paint that too. We are going to put down carpet tiles so they can be easily removed and even cleaned it need be. This smell coated the walls like paint! What an ordeal.