Hidden units

[Mowgli]

Hidden units are not revealed by the recon spotting ability.

Perhaps a noob question, but .. what units does recon spotting reveal then? Is there a non-spotted but non-concealed status?

Recon spotting also allows you to discover enemies (who are not revealing themselves to you by opportunity fire) in your own turn while you're on the move. 

As in all Tiller games, you usually only reveal enemy units that are within LOS of at least one of your units AT THE START OF YOUR TURN (and who are not concealed in PzBattles/SquadBattles). This means that if you move around corners/over hills into hitherto un-reconed terrain, you're basically moving into the unknown. Enemy units are not "updated/revealed" live while you're moving. 

With recon units, you can just crest the hill, use the spotting ability and see if everything is clear and safe for your column to continue. You can "spot on the move", so to speak.