Assuming she has, which isn't certain but is pretty reasonable, the bugbears probably don't recognize her significance to Team Evil. I assume Serini, mildly paranoid high-level rogue. doesn't introduce herself and explain her backstory to every backwoods shaman who she buys alchemy supplies from. (Or if she does, she gives a fake name/backstory.) It also helps that, if Xykon and Redcloak were going to expect Serini, they'd expect her to have been resurrected, not healed with troll blood; even if Oona mentioned a halfling trader with a weird troll-face, Redcloak would have no reason to think that that halfling was the high-level Gate-watcher Serini Toormuck.