If you don't quite see what we're talking about here, though, lemme point it out:
Yep, it's a raven! (Or crow? Sorry IDK how to distinguish the two.)
But why does that mean Bran was listening, you say? Well, if you recall, Game of Thrones has shown us many times over the years that Bran can see through the eyes of animals. It's part of his abilities as a warg (someone with the ability to enter the minds of animals).
And, most recently in Season 7, Bran's been using ravens specifically to ~spy~ on the Night King and the White Walker army.
Although the Night King was definitely like, "I see you."
So, not that they know (yet), but everyone in Winterfell should be suspicious of any ravens creeping around their conversations!
On top of, ya know, being creeped out by Bran himself...