I thought thieves were supposed to treat you differently when you were wearing Nightingale armor. I just ran into a thief (I was looking for one actually) and they treated me no differently than usual and gave me no unique dialogue options. Is this just a myth about Nightingale armor that theives are supposed to treat you differently?

Are you the guildmaster?

Once you do the Thieves Guild Radiant Quests and become Guildmaster, then they start treating you differently in my experience.