Thank you for checking in with an update. While you say that your parents are "still fully functional, for the most part. They are in their 60s and work full time." Some of the behaviors you have described in your first post, may indicate the need for some medical attention, especially if your dad's "ranting" and losing things is a new symptom.
It can be difficult to face the loss of cognition and/or a personality change in parents. There can be many physical causes such as diabetes, heart problems, etc. It might be a good idea to compare their behavior now to where they were a year ago and suggest a medical check-up.