The ECG shows the electrical activity of the heart. It says nothing about the structure. It certainly can NOT identify murmurs as these are due to turbulent flow through valves or other structural abnormalities. It can suggest that the heart is abnormally enlarged and can suggest some structural abnormalities but it cannot diagnose these.
Its primary benefit in routine medicals for people who have no symptoms of coronary heart disease is in confirming that the patient has a normally conducted, regular heart rhythm (sinus rhythm) and that there is no evidence of old damage to the heart (from a 'silent' heart attack - yes, they do happen!). Damage from a heart attack shows as unusual routes and directions of electrical activity as the damaged bit doesn't conduct electricity normally.
Irregularities of heart rhythm (arrhythmias) are often asymptomatic and some only show up when the patient collapses for the first time. ECGs can identify some of these abnormalities before they cause problems and restrict flying in those who have them (some aren't allowed to fly solo for instance, rather than being grounded altogether).
Angina does not show up on an ECG except during an attack or when the patient is exercise stressed (like doing the fitness test!). Some types of angina don't even show this change. ECGs are therefore not a good test for coronary heart disease on their own.
Essentially ECGs are good for identifying abnormalities (if the ECG is abnormal then there is a good chance that there is something abnormal with the heart) but poor at ruling abnormalities out (the ECG can be normal with a very abnormal heart). The machine is not very good at identifying abnormalities and tends to the side of caution. If the machine says the ECG is normal then it probably is. If it says the ECG is abnormal, that doesn't necessarily mean there is a problem!
Hope that makes some sense. Apologies for the long post
Bob