If the rest of your buildings are running smoothly, I'd say the only common denominator is that TBT PM.Another idea is to send a "secret shopper" to go through the tenant onboarding process on these vacant units.Yet another is to see if you can reach out to your remaining tenants (under the guise of a customer satisfaction survey, for example) to see if they can shed some light on it.

What are the most common commercial properties types lenders lend on? 

That is a common issue.  

It got tricky to do it on the phone the first time. 

@Elizabeth Wilson  its far to early in the process.. having done this for many years before it became ILLEGAL in our state the only way we bought them was within 48 hours of the auction and usually the day before.you need big time cash and big time risk and you need to have in at title company that will give you a date down on the phone.. if your in a title abstract state that takes for ever to run title I don't see this as any kind of viable.

You will find that it's common on BP to recommend that you don't hang onto no-mortgage houses just because they cash flow nicely that way.

We were complaining about the bad cell reception in our house so the cell phone company told us they could put a tower on our land for $800/month. check if that land generates income or can generate income for you.

In an apartment building, the landlord is responsible for the HVAC unit(s), roof, water heaters, toilets, exterior of the building, windows, common areas, landscaping, etc.

How do I go about getting the owners current address, phone number, etc.

@Josh Caldwell, It's commonly called a consolidation exchange.