Way too overpowered.
First, the discovery chance needs to depend on unit size - much more difficult to hide 100 men than to hide 10. Also, cavalry and units with siege engines should be harder to hide.
Second, I don't think a combat advantage fits. Medieval battles weren't fought that way, they were usually set up for hours before fighting ever started. Plus, having more enemies than you thought you would will usually be enough penalty already. The only effect I see is a moral penalty, and then only if the change is significant (i.e. you face twice as much enemies as you thought you would).
