What steps do I need to go through to identify a foul odor in a single a room?

I have an odor in an enclosed room in the middle of my house. The room has a wet bar and sink in it. It is adjacent to a water heater closet and a bathroom. This room has a skylight that was repaired about 6 months ago for a water leak under its flashing. At times the smell seems to originate from one corner of the room. Other times from the discharge vent in the ceiling. I've broken down the plumbing from the sink and don't think the odor is from the sewer. I see no visible signs of mold. I've been in the attic and nothing obvious or no odor. I have a concrete foundation. This room is tiled and not carpeted. No apparent entry way for animals in the walls or attic. The smell is like a dirty fish aquarium and isolated to this area. I live in the DFW area. What's next? Thoughts? opinions?