Could w/ B be with Barty? As they were the ones who were chased off by Barty and didn't directly challenge Dolohov.

I considered it could mean whereabouts as he knows they were part of Linus , Cedric and Katie's distraction posts and wants to work out where they are, but I think the Barty reference is more likely.

ETA : while I still think the above is most likely, could also be a ref to working the Bracelet trace, as only someone in same social circle as Selwyn was likely to know how.