Yep, if a unit has that little tree symbol on its card, it means it's invisible to the enemy. And yes, you can fall victim to this too--against both the AI and humans players online--so beware! When you have battlemaps that has woods, it's not a bad idea to use light cavarly (if you have some) to scout the larger groups of trees to make sure the enemy doesn't have some nasty surprises waiting for you.