What I expect, personally, is that Serge (and Sim) will walk down the street with the detect spell going and pretty much note where they might be. The other end of things will have to move through the sewers and they already know that wights are in the sewers in numbers. (They killed over 30 the night before). Neither may not encounter anything before the house is located, it's just a back-up plan. If either finds something, then they can send for the others or backtrack to a given place at a given time and plan for an action.