The perception barrier may not have blocked the sight of Watchers and Balancers, but it did provide one additional feature that was just as helpful.
That feature was blocking out the noise that was coming from within so that it would not reach outside anymore.
Balancers, Watchers and Spirits could see inside if they came across the perception barrier, but they could not hear anything inside until they stepped inside.
The only issue that Ash had with the different types of barriers was that they could not be used when starting combat.
It was a method to help reduce interference and to trap or keep out others, but it was only useful when a plan of action had been created in advance in a way that would be more suited to trapping a Spirit inside before it could get away.
If she had no prior knowledge of the Spirit's location, it would have been very difficult to set up that type of barrier all around without being noticed.
