So ninja can't spot enemy hiding in ambushes?

[homefry]

So there was enemy army that was sabotaged by my ninja. Next turn it disappears from the map in some light trees/bushes.

I sent my ninja to walk all around there and could not spot the ambush. Next turn, the enemy proceeds to attack my castle.

[Xatham]

Depends on his skill at spying. He can miss it if your ninja isn't epic.