You hear about both cases a lot- the ones that no one suspected and the ones that some people knew were at risk. According to the website livescience.com between 50% and 75% of people who attempt ctb talk about it beforehand, but it would seem to be very difficult to get reliable statistics on this, so who knows what the percent really is. Cultural factors may play a role- where it is more taboo people would obviously be less likely to talk about it first.