I think you're getting it- the key is going to be what the provider documents in the assessment and plan. That should give you an indication of how it factored into the medical decision making. Just because a patient comes in with a certain chief complaint (headaches for example) doesn't mean the provider won't find something on the exam and factor that into the assessment and plan. This could increase the overall complexity of the medical decision making, and in turn affect your code selection for the E/M level.
So in my example above, you may have something that is not in the History, but is documented in the Examination and Assessment/Plan (Patient presents for nausea, on exam they are hypertensive and have an arrhythmia), in which case you could count that towards the level of EM service.
OR
You could have a scenario where information is provided in the history (patient presents for leg pain, upon gathering history we find that the patient is diabetic, hypertensive, and also depressed) but nothing is necessarily seen in the Examination regarding this info, but it is still listed in the Assessment and Plan. In this scenario, you can also count that info as part of the E/M level.
So using these 2 scenarios as examples, you can see that you don't need to see information in the History, Exam, AND MDM to count it. The most important thing is you need to see it in the assessment and plan to see how it factored into the medical decision making.