I've seen these before, and I hate them with a passion. Can't see it if you look directly at it, but the more motion-oriented vision at the edge of my vision would catch it.
They seem to be rare, though.
Peripheral vision is much more sensitive to higher frequencies. The central vision taps out at around 40 Hz, while the far peripheral can sense up to around 120 Hz. Doesn't mean that the sensitivity is zero above that, though, just diminished. So any self-respecting system engineer would set the modulation frequency to twice that. Sadly LCD monitors are apparently not designed by self-respecting system engineers.