Subarmalis designs?

[MD]

Of course if these are under armour garments then one must immediately ask, why can we see them?

An answer can possibly be found in Roman literature. Septimius Severus made the Praetorian Guard parade only in what was described as a subarmalis before disarming them. SHA, Severus, 6.11 If this was done so as not to arouse suspicion, then the Guard must have frequently worn these garments without armour.

In Tacitus works the presence of light auxiliary infantry cohorts at various encounters is frequently mentioned. Since there seems to be no specialised light infantry cohorts in this period and most auxliaries are depicted as heavy infantrymen I suspect that these were soldiers from ordinary cohorts fighting as light infantry when required.
I doubt they had two sets of armour, but wearing just their subarmalis without the heavy mail shirt seems like a good compromise between mobility and protection. The same should be true for other parts of the army.