FYI, the Logic Board must be removed in order to replace the Battery because Apple considers the battery to be part of the Top Case/Keyboard Assembly (the battery is glued to it)....actually every single internal component must be removed to change the Top Case/Keyboard/Battery Assembly. So it would not surprise me if a tech discovered something else during the repair.
Over half of the laptops I repair have had some sort of accidental liquid damage. Half of those people swear to me nothing ever happened, some would later fess up. It is always possible someone else accidentally spilled something and was too embarrassed to say anything. All it takes is one drop of liquid at the right place, it doesn't need to be a massive spill directly on the laptop, it could be just a splash from several feet away. Sometimes the laptop may appear to be fully functional.
Apple has a very strict policy....if any signs of liquid or corrosion appears on a part, then the Apple tech is forced to replace it. Apple is after giving a person a fully functional device (Apple doesn't care about repair costs). If you work in the service industry interfacing with the public, you quickly realize you want everything to be perfect so that it minimizes the chance of the user complaining about any repair issues. I've had people I tried to make some exceptions thinking I knew them well enough & they understood what I told them, but they quickly forgot my warning about the shortcut on the repair. So I can understand Apple's policy.
