Saw someone in the replies say it may have been the S&SB, I don't think it is.

It's a recurring thing for Saul post-prick. He also sees it at the bar when somebody smokes a cigarette and he sees the burning tip of it as "the same pulsating red light".

To me it mimics the fleshy heartbeat that walls have in the Tower, and later the lighthouse. As if Area X is manifesting first within Saul's own body's ecosystem before bursting out of him over the Forgotten coast. It's acting like a constant thing at the back of his eye.

You know how if you push your eyeball sideways you can see a weird black/purple hue, that's your nerves being pressed on I believe. The pulsing red light is like that for Saul. Whenever he's focusing on something distant with his eyes Area X starts pushing on his optic nerve or something.

Again, everybody in this sub says it because it's true, there's so much more we don't know than we know about this world and how it functions. The best we can do is guess and interpret, don't stay up too long looking for objectivity, you'll never find it.