Is the bird sigil from Geass users visible? In the beginning I assumed that it wasn't, however later when Suzaku forcibly takes Lelouch to Charles, Lelouch for some reason notices, that the Emperor also has Geass, which makes me wonder if there really is a sigil that is visible.
IIRC, Geass is invisible and undetectable save for to other Geass users and the beings that give the power of Geass to humans. I can't recall what they are called.

This is further proven in R2 when has to deal with the guy who has the Geass canceler. Again Lelouch and his brother Rolo were able to detect him.

I kind of watched a few episodes recently but I did watch both series some time ago.