Silent field loss. Glaucoma in early stage is asymptomatic. Glaucoma if undetected and untreated can progressively damage the optic nerve and eventually resulting in irreversible visual field loss. This is why it important to have a dilated retinal examination and intraocular pressure check, especially after 50 years old.
Yes. Glaucoma can present in different ways, but is typically asymptomatic until late stages, thus many people have 20/20 vision and don't know they have glaucoma ("silent thief"). It's a slow progressive process that can affect one eye first (the other eye can cover for the bad eye), so perfect vision does not rule out glaucoma.