Yankes, I don't want to be too greedy, but what do you think about introducing reduced spotting distance for some units? I mean spotting them at distances that are closer than seeing range, for example 10 tiles instead of normal 20 (per day).
This would probably be an armour feature. I think it would be best expressed in decimals; so stealth: 0.5
in the armour code would mean that you can only spot them at half the normal distance.
Obviously this would enable stealth units (ninjas) without any dirty tricks regarding invisibility.
In a more advanced version, stealth would be a derived stat from the unit's TUs, Reactions and maybe Melee Accuracy, and also with possible bonus from armour (dark clothes, stealth fields etc.) But that's probably too heavy, at least until introduction of adding new custom stats.