By July of 1969, there would have been millions of the Silence around. The Doctor gave them a chance to run, but where could they have gone? They had no spaceships, and no need of them - the human population had no clue up until that point in Time that the Silence were even among them.
So even though the humans were given a post-hypnotic suggestion to kill the Silence on sight, there may be still millions of the aliens on Earth Prime-Time.
That means they're still out there, in every TV show we watch, and have watched since 1949 (farther back for the British). And that includes the Westerns, the sitcoms, the medical and courtroom dramas, cop shows, the historical dramas from the Greeks and Romans through the Shakespearean age into the Victorian age and on throughout the 20th Century.
So... anytime a character turns around quickly for seemingly no reason in those earlier shows, they may have just seen a Silent. And in any TV series in which a character not only spins around but also fires a gun at something they thought they saw, then we can assume that they just shot and probably killed one of the Silence.
If you can think of any good examples, let me know!