Even if they get some of it off, it returns in 2-3 days.We're in the northwest where it's rainy and damp all year long and as far as I can tell from investigating the internet and youtube (yes, the source of all truths lol), these are mold and yes, they are hard to come off.

They said it was pretty bad but I haven't had a chance to investigate myself.

It's a bit of a nuisance when they lose a toe or a few fingers and the investigation shows the person was also your tenant especially if the regulations state the landlord is responsible for upkeep..

Now at least I have some options to investigate 

This is worth investigation by all landlords.This "IS" should not be confused by the Clinton definition of "is":)

I looked at sub-metering a property that I own and although I have decided against it for now, the investigative process was educational.  

Do it yourself and investigate old addresses.  

I want to investigate this ASAP and assist in any way I can with your closing. 

Do some investigating on your own and findout from the city/county how long is the permitting process.  

Rely on and follow your internal procedures to do a thorough investigation, particularly into his rental and work history.