Since the mithral versions of a chain shirt and a breastplate (see Mithral Armor in the DMG) specify they can be worn under normal clothes, I had initially presumed this to be a feature unique to these magic items. However, upon reading the description for a chain shirt:
Made of interlocking metal rings, a chain shirt is worn between layers of clothing or leather. This armor offers modest protection to the wearer's upper body and allows the sound of the rings rubbing against one another to be muffled by outer layers.
It seems to suggest that a normal chain shirt can also be worn underneath other layers, but doesn't specify 'normal clothes' like the mithral chain shirt does. Can a mundane chain shirt be concealed under light clothes like a shirt?