To have a thought or idea, and forget it right away. As if you came up with the idea while staring at one of the Silence, and then looked away.
Coming from Doctor Who
, where the order known as the Silence are "memory-proof", so you can only remember them while you look at them, and you forget them when you look away. The Silence uses post-hypnotic suggestion to make people do their bidding without people realizing what they're doing or why.