In Doctor Who, the Silence (from the Impossible Astronaut 2 parter) were able to edit themselves out of a person's memory. However, surely when a normal human saw one they would scream? And then wouldn't surrounding humans wonder why this person was screaming, take a look and scream as well until everyone realised they were there?

Also, if they've been around for so long, why haven't we seen them before, say in the episode where the Daleks are in the Empire State Building when its being built?