1. In my current game they told me, I actually succeeded in establishing communications with them. I haven't a whole lot of experience, but in past games I've identified them by the fact that they hang out at planets without any populations.
2. I have no idea on percentages, but I haven't had even a short game without meeting some.
3. I have found evidence jump drives from salvaging their wrecks in my current game. I've never seen any jump, though.