What do I do about other complaints during a workplace investigation?

What do you do if somebody brings up other workplace complaints during their interview with you about a separate issue? If it's a very, very unrelated issue, you can ask the person, "Do you understand how to make a report in your investigation appropriately?" Usually, by the time I've gotten into an interview room, I'm very well aware if a client has a reporting hotline, a comment box in the break room, or how they accept reports. Typically, my time is budgeted by the organization to a particular allegation and the interviewee needs to begin a new and separate process by starting at the beginning with proper reporting.

I only make a note to inform my client contact that there may be an additional issue. And I will report that back up to my client contact because I wouldn't want to be responsible for having been negligent on behalf of my client in receiving a complaint that was never furthered. And so sometimes the client will follow up and say, "Hey, Natalie said you had a complaint about this, that or the other. I wanted to make sure that that was going to get into our reporting hotline," or, "Can I ask you some more questions about that?" But I don't, as the investigator, take on broadening my scope of investigation because somebody else in an interview position brought up some other allegation.