Repeatedly on HN, this will happen. Someone will mention something, like for instance someone says something about a16z, and the next thing Mark Andreessen is commenting back in the thread.
Or someone says "X company is bullshit!" and 5 minutes later you see in the thread, "CEO of X Company here.. We're not bullshit".
Some of these people are famous and I would think, not hanging out staring at HN (in these particular threads no less) all day.
How are they being alerted that their company etc.. is being discussed and able to respond so quickly?
1) They are, in fact, hanging out at HN more than you think.
2) They routinely type their names into the search box, out of curiosity or to gauge their publicity.
3) (unlikely) They're continuing the age-old trend of kibozing (see https://en.wikipedia.org/wiki/James_Parry), where they write scripts that crawl and grep for certain terms, so that they may respond. I'm not sure how HN handles bots, and if anyone actually does this here.