I don't know if this has been discussed but I'm a little confused.
At the end of Old Guard, a drunken Booker enters his apartment and is greeted by Quynh.
I did some research and as near as I can tell, Quynh never met Booker since she was imprisoned in the early 1700's (if in England) or between 1692 and 1693 (in the US).
Booker got his immortality in 1812.
So, how did Quynh know him much less where to look for him since their paths never crossed?
There's a dialog in the movie "over 500 years in a box will make anyone crazy". So she was definitely boxed around early 1500s to maybe even late 1400s.
However, they also have visions of each other, especially when a new "immortal" spawns (somehow). Perhaps they also sporadically share visions and memories. So when Booker and others were found, Quynh was alive and heard his name and saw his face. Plus they also seem to be drawn to each other somehow so they're aware somewhat of where they are.
She must have escaped perhaps recently and been tracking them and watching them. All this means, she knows the dynamics and what's going on - suddenly she finds one guy separated from rest so she showed up tracking him to find out why