Seth was forced to leave his improvised laboratory and fix the window of the carriage. A small hole in the glass was neat, without any unnecessary fracture. A great magic force was used, but that confused him even more. He couldn't feel any remaining many.
If the window is hit by the arrow, the glass will be shattered. But this small opening indicated a precise use of mana from inside, as it leaves no breach on the protection barriers he has set. The only persons on the carriage were the weak Vincent with a hardly sensible amount of mana, and the strange woman who joined their journey.
But what made him suspicious, was the fact that there was no magic coming from her. Even if she hid the demon's core somewhere, Seth would be able to sniff it off, which left him with only one option. The person responsible for that hole was Vincent.