There is a very big difference between being on the outside looking in, and being on the inside. At least, for most people.
When Im really suicidal? Nobody knows. Because my affect is outstanding.
When I'm struggling with ideation? Then people usually know. Some, anyway. Okay, not a lot. But that's because I isolate like a mofo. The point being, though, that I'm in pain. It's written into and onto me, and I'm oozing that onto everything I touch.
Other people are different. And some people do just snap with no warning whatsoever... At least in their own minds (some of this is from school, others were fellow 'Self destruction, full speed ahead' friends). There are usually a whole street parade of warning signs (self destruction, ho! Being one of them). In my experience (biased & limited) these are usually from people who have not a lot of self understanding, are deep in survival mode, and don't consider suicide as "ever" being a viable option. It's one reason why coming up with safety nets and plans (the what to do when suicidal, not how to commit) is a smart thing to do in advance.