Quote Originally Posted by HoreTore
The quite obvious answers is:

Don't place your armies in ambush positions bordering neutrals!

The only reason I can see for someone to have armies in ambush positions, is when you fear an invader. And then if someone invades, isn't that a good thing? If you wish to frighten a would-be invader, or stop neutrals going through your lands, you'll have to put your armies so he can see them, and blocking the road to your settlement.
the other choise you have is to build forts if the army has depleted its movement points .. that way your army is visible to AI and you do not have to worry about ambushing others ..