Similar to how when you hold Alt, and units have their positions and facing revealed, if technically visible, also include an indicator that says if they are in day or night visibility.
While for vanilla this doesn't matter, because Aliens all have 20 vision, in mods, like Xpiratez and The Xcom Files, enemies have variable sight ranges like Xcom does, and so being concealed is important. However, it can be non-obvious whether you are illumiated or not, because light falls off, but there is no solid delimination between "Visible to Day range" and "Visible to Night Range"
