The Overrun maneuver in Pathfinder allows a creature to pass through an enemy's square as part of a charge or move action. If the attacker isn't detected by the target - if it's invisible or just unnoticed - can the target still avoid the Overrun attempt and allow the creature to pass through its space unimpeded, or is the target forced to contest the maneuver?
Looking over the rules it doesn't say anything specific, just that the target can choose to avoid or not. However, it does say that the attacker needs to enter the target's square before the target chooses whether to avoid or resist. As a GM I would say entering the target's square is enough for the target to notice the invisible attacker. That being said, I wasn't able to find any additional information either supporting or refuting that.