Unfortunately it is quite common for people to commit suicide without mentioning their plans to anyone, without anyone even suspecting. For example during our training we were told of a college student who committed suicide in his dorm room. No one missed him until the smell of his decaying corpse disturbed the other students on his hall.
It's also commonly asserted that suicidal people attempt suicide to manipulate others, for example to win sympathy or to send a message. In my experience that is not the case. I've met lots of suicidal people; a former employer of mine, Chris Schefler - a cofounder of the world's first web hosting service - actually did commit suicide. In my whole life I've only met one person who attempted for reasons of sending a message.
"Self injury" is a different thing from suicide attempt. That's commonly practiced by people with Borderline Personality Disorder. A psychiatrist explained to our training class that people with BPD do not experience feelings, so they cut themselves in hopes of feeling something.