I'm not sure about that,i think that is another figurine, it vibrates because it senses high magic , quite normal considering the residents .
I don't think Triss is in the figurine , in that case it means it was compressed with Philippa consent ,otherwise it makes no sense, Philippa would have sensed her presence , when you cross the mist for royal blood the imperial ambassador is saying that his sorcerer sensed Triss approaching so Philippa not sensing Triss in her home is out of the question .
My interpretations was : Triss flees the troll and arrives to Vergen during the peace negotiations and goes to Philippa house where she encounters only Cynthia who casts the spell on her and immediately dispatches a messenger to the Nilfgaardian camp with the figurine . Unfortunately for the guy , the mist emerges and he is wounded almost when he was about to leave the battlefield area and dies a short distance further , where Geralt finds him afterwards . Fearing she might be revealed , Cynthia flees to Loc Muine when Geralt crosses the mist . On the Roche's path she seems to cross the mist herself with the figurine .
Another interpretation is that Philippa knew all along that Triss in her house and Cynthia betrayed her right when Gerald decided to cross the mist and Philippa leaves Vergen to help him . Fearing Philippa would sense her presence, Cynthia sends a messenger with the figurine , in this variant i assume that guy runs like hell and crosses the mist before Geralt (pretty unconvincing) .
What do you think ?