We've suspected for a few weeks now that Bran's visions let him communicate with others on Game of Thrones and this Sunday's episode pretty much confirmed that theory to be true. Thanks to a little late-night rogue vision quest, The Night's King actually saw Bran and even branded him with his mark. Up until now, watching others in these visions has just been a one-way street, so how was The Night's King able to see Bran? Right now your guess is as good as mine.
Back when Bran visited the Tower of Joy, it seemed like Young Ned Stark had heard his son cry out to him for a brief moment before heading inside the tower. That alone was a surprise twist, considering visions usually allow the person to observe but not interfere. But, now it seems as though Bran can also make himself visible during these visions as well? This could be an indicator that he's getting stronger. Perhaps the more he practices at becoming the new Three-Eyed Raven, the more power he can wield in these visions. Or maybe it's just proof of how powerful The Night's King truly is. Maybe only he is able to sense Bran's presence because he himself is a mystical being?
The truth is, we don't know exactly why or how The Night's King was able to see Bran. Remember, this is Game of Thrones we're talking about where answers are seldom and death is everywhere, so the writers aren't going to make it easy on us. But, it does add a curious element to Bran's storyline and could even indicate that he shares some sort of strong connection with The Night's King. Was The Night's King once a Stark and can sense when a fellow blood relative is near? Or is the House of Stark his No. 1 enemy to the point where he's developed a sixth sense of when they're nearby?
Like I said, all of this remains a big mystery, but I doubt it'll be the only time Bran will across this blue-eyed villain. He knows Bran is out there now, and he won't stop until the job has been done.