They of course noticed and certainly tried to reinforce that message, because they indeed became aware of the split. But the initial reason was a lack of trust in media and domestic politics and not some external propaganda channel.
And expect this to get much worse if you now increase surveillance. That said, NPR just suspended a journalist that did notice some form of propaganda from domestic sources, which might explain why people were distrustful in the first place.
In fact, you might be a victim of propaganda. Maybe read up on it.