As was stated the ammeter will show how much current is going into or out of the battery. If there is an extra circuit added to the car it should be on the ign switch side of the ammeter which would not show on the ammeter if the alt is putting out enough current to supply it at idle and if the alt is not putting out enough to supply it at idle then it will draw from the battery at idle which would show a discharge not a charge. Basically if your gauge is showing a charge it is telling you the battery is taking in some current at that time. Even a fully charged battery will draw a few amps to keep it at full charge and most of the stock ammeter gauges will be centered or just slightly on the charge side when all is working good and the battery up to charge. I agree that what you are seeing sounds like a battery problem as I have seen it many times over the years.
Also your 14.9 volt reading you got at what temp was that as it will have a higher target voltage as it gets colder ? I have seen it as high as 15 and even 15.2 volts in 20 degree temps. Course at 70 degrees it should be around 14.2. Ron