Warning: If you have a fear of rodents...

Then you might want to skip this entry. In removing the plywood floor in the utility room, I may have uncovered part of the reason for the smell.
Here is a glimpse of our new friend after removing the plywood to revel the palettes that were holding it up:
Our friend and his nest:
The disinfected area:
(I placed a wood barrier on the left hand side with a layer of tar paper and two layers of that panel insulation.)
Finally I built a makeshift support for the water heater, because I plan to cut the shelf in half and remove all the crazy random boards they used to patch over the outside wall on the right.