Like Soly i do not encounter this type of problem usually (conceilment). I am carefull not to hide units in areas that are in the path of the attacking AI army and use some pawn to show the right way. I also adapt to the climate some rain or light fog reaaaly helps in conceilment.

I do notice that the second time around (in the same game-same map), the AI will look in the area that i had previously conceiled a unit.