My understanding of this (which is just an opinion of events) was that it was initially assumed the group were just passing through. However, once it turned out they were staying the barman's attention was piqued. He makes some comment about the near hostel/camping stretch being some 40 miles away. I assumed at this point that if they had been there, they'd have been able to finish their drinks and leave.
However, once its mentioned they are staying locally, at Ravenswood Cottage, I thought it was the sign that the barman now knew they were never going to leave the village and would be killed very soon.