I believe this sort of thing's been discussed in the past regarding a known issue/bug with invisibility & sanctuary spells not always functioning properly due to the way Bioware wrote the code, and it's not something the Layonara team can fix (to my knowledge). But realisticly, snakes use smell and thermal sensing as their primary means of hunting prey, so they wouldn't need to see you with their eyes to know you were there and strike at you.