In Wargame: Red Dragon (the spiritual predecessor to Wargame by Eugen Systems) units in trees, or buildings, or cover in general were hidden until another unit got close enough to spot them. Recon units could spot them from further away than regular units. So yes, it was possible to ambush or flank enemies, and counter those moves with proper recon and spotting.
Although recon units could only do so much. If your enemy was deep in a forest, or inside a town your scouts couldn't see them because obviously there's no line of sight. So you had to send in pickets of infantry to find the enemy and root them out.