Hello Kris.
This is a common challenge encountered by business analysts. How can a business analyst ensure that ANY discovery process yields complete results? The short answer is to use multiple complementary tools and techniques to approach the problem from multiple perspectives where each validates the other and fills any gaps in your discovery process.
In your case, you should focus on Processes, Business Entities, and Data Flows within the scope of your domain or system.
Let's take a moment to understand what processes really do. Process take inputs, do something to add value, and produce outputs. These inputs and outputs can be data/information, products, or work deliverables. There is a well known diagram, called a
SIPOC diagram, that reinforces this concept. SIPOC stands for Supplier, Input, Process, Output, Consumer. The supplier is where the inputs come from. It might be a database or worker and it could be internal or external to your organization. Those inputs flow into a process. The process does stuff to add value and then generate outputs which are consumed by a person, another process, or database for later use.
Some of your processes will be easily identified on their own, but by focusing on the information that is important to your organization you can discover other processes that you may have missed. Business information/data is almost always an input or output of a process. Otherwise, why does the information/data exist in the first place?
A Business Entity Model (also known as a Concept Model or Logical Data Model) can help you document and understand the important information or "things" of your business. For a mortgage business, this might be things like Borrower, Loan Application, Credit Report, Credit Decision, Underwriting Decision. Each of these business entities have attributes which describe them. For instance, the Borrower entity may have attributes of First Name, Last Name, Phone Number, Date of Birth, etc. For most of your discovery process understanding the business entities is enough. Though it may be necessary to document some of the key attributes as well. A great place to discover the business entities and attributes of the business is to review APIs and database tables. Of course, this will show you the physical data structures. These need to be translated into logical data structures that reflect the way your business thinks about and uses the business entities and attributes. But APIs and databases do a great job of helping you find all of your data. Remember, data exists for a reason. Data will help you find your processes.
Data Flow Diagrams can then be used to show how information/data (Business Entities) flow as inputs into some processes and outputs from others. The Data Flow Diagrams connect it all together.
By using an iterative process, you can document Business Entities which, by virtue of being an input or output to one or more processes, help you discover other processes you may have missed. And you can use processes, and the inputs and outputs of these processes, to discover business entities you have missed. With each iteration you fill gaps that you might otherwise have missed.