If the Silence have been around throughout the run of 'Doctor Who', there may have been another episode in which their presence might be suggested.....
In "School Reunion", we were introduced to another alien race who thought they could take over the planet - the Krillitane.
The Krillitane had the power to absorb the physical features and abilities of other species, usually those they had conquered. For example, Mr. Finch seemed to prefer remaining in his human form even after school was done for the day.
But the Krillitane may have also absorbed something from the Silence who were in their midst, observing them.
Here are the Silence at rest:
And here are the Krillitane as they snoozed......
The Krillitane probably thought it was a talent they picked up from some of the humans. After all, they only saw the humans during the school day, not as they slept. And like the humans, the Krillitane would be unable to remember if they saw a Silent.