Remember that since they are blind, Falmer are alerted by noise, not sight.
Hmm, lore-wise maybe, but gameplay-wise I think they are the same as any other NPC, I tried sneaking by them several times, even if I am stood still and not moving, they will detect me as soon as I am in their line of sight, even when I am wearing muffled boots and not even moving anyway.
(I never perk into sneak though, but I can sneak by most bandits using the same gear, as long as I keep out of line of sight, thus I assume Falmer aren't really blind)