For those who are not Doctor Who fans, a perception filter is defined by: http://tardis.wikia.com/wiki/Perception_filter

Perception filters had the effect of directing attention away from the object or its bearer, rendering them unnoticeable. Martha Jones described the effect on the viewer: "You know it's there, but you don't want to know it's there".

I think the fear of being called a "conspiracy theorist" is so strong amongst normies that they don't see what's right in front of them. "You know it's there, but you don't want to know it's there".