The Master's head snapped up in surprise, looking upwards at the Doctor. He stepped out from beneath the floor and leaned against the railing at the bottom of the steps so they could look at each other properly. Instead of demanding to know why he'd run, the Master decided to try a different approach. "Are you alright?" He asked, genuine concern showing in his eyes for his old friend.
Whatever had happened, it must've been serious to cause him to leave all of them without so much as a word. And if the Doctor had discussed those reasons with Missy, she hadn't shared that information with her younger self.