The falsely innocuous text - because Severus is the only Death Eater to be familiar with and have the capacity to adapt to Muggle tech on his own, so of course he notes it with ease - decides his course of action for him. Why does he need to check in with anyone if he doesn't have anything planned? He steps back and clenches his fist, and prepares to do something he's not done in many years, and not wanted to do, ever.

He stuns the dog first, silent, barely flicking his hand in its direction to do it before he grabs at the older man's left arm, fingers wrapping vice-like around his wrist and jerking him forward as he hisses the spell to compel a Dark Mark to appear. It's easy and like breathing but it's still a brutal bit of dark magic that most Death Eaters themselves have never had cause to use. (Guess who usually does. Go on, guess.)