Good move using a service for your first note. Assuming they are doing all of the tax and title level DD, including review of the note and assignment chains, I would recommend the following additional things to look at.
Borrower Correspondence - These communications, especially those from the borrower to the lender, can give you important clues about the emotional equity the borrower may have for the property. For example, if you see that the borrower has expressed interest in or applied for a loan mod or forbearance, that's a strong indicator that they would like to stay in the property.
Demand Letter - If a demand letter is present in the collateral, it means a default has been declared and the statute of limitations for the state comes into play. The timer starts with the lender's declaration of default, or the with the last scheduled payment, whichever is earlier. Generally speaking, the borrower must start foreclosure prior to expiration of the SOL, assuming there are no mitigating factors such as a bankruptcy stay. (Your DD service should actually be looking at this.)
Those are the key things that come to mind for me right now. Others may chime in with more.