This is going to sound cynical but I'm guessing quite often people act surprised to cover any guilt they may be feeling or to ensure others don't blame them for not knowing. I only have myself as a reference point so this will sound a little generalised, but I can't imagine if someone is so depressed and resorts to ctb, that they could have 100% hidden that, especially in every day interactions with people they live with. People who are close must suspect or know in some way, but maybe they didn't say anything, maybe they didn't want to find out