Talkative
One overlooked sign of suicidal thoughts, is people becoming more talkative. They'll often talk about your future and also good things and happiness in your future. They'll be excited with and for you.
However, they often won't show any kind of negative, "feel bad for me" signs.
You'll basically have to read between the lines in order to recognize their thoughts and save them.
A star shines brightest when it's dying.