The termite and moisture inspection didn't even mention that particular floor, and I had another mold remediation expert come in for a full look-through, and he didn't note any mold there either.

If it's an older home from the late 60's and prior, it could have cast iron sewer lines which tree roots can penetrate looking for water.

His recommendation to me is prior to renting your unit out, get documentation of moisture readings on the floors, walls, and air.

I am assuming I need to hire a water leak detection company to use some sort of moisture detection equipment.

The joist has swollen from all the constant moisture. 

It was built in the 1960's and apparently the moisture has caused the flooring to rot and has given way.  

No visible water, probably just moisture coming in from the foundation. 

My main concern is moisture and mold.  

Remove any loose paper and re-apply water till all layers are off.Use towels to keep the base boards dry or they will cup from the moisture and separate from the wall.The key is keeping the wallpaper wet.  

Real bad.So who knows the best way (aside of torching the house) to get rid of cat pee smell - which I fear right have penetrated the hardwood floors.