I know Number Six did something to Baltar and that's how the phantom Cylon agent appears to him and can't be seen by anybody else. Why doesn't Helo see the Cylons that are monitoring Him and Boomer as they travel on Caprica? The Cylon overseers just stand there silhouetted against the skyline and he never sees them. I know they are there, because they put a beating on Boomer in episode 9 of season one. Is this explained somewhere and I missed it?
Helo is on a miserable, irradiated planet. He is dealing with fallout-precipitation, extreme hunger and fatigue, and he is moving by night. His attention is possibly not a hundred per cent.
Also, this is probably one of those situations were what we observe as the audience is a convenience - we need to see that the cylons are tracking and observing Helo. And while we can obviously see them, it doesn't necessarily track that Helo will see them from his perspective.