Kevin was a realistic person. He had worked as a scientist for so many years, and he wouldn't easily believe in what he heard without seeing the real situation with his own eyes.
When he was working in the laboratory, he was willing to work overtime just to prove some theory or some rumors related to what he was working on. He would try to experiment with things without caring about his tired body just to find proof of some theory or rumors.
Some people indeed believed in their instinct, but for Kevin, he would still use his brain first before believing in his instinct. It wasn't like Kevin would completely ignore his intuition, but he would connect his hunch with the logic in his head.
He would only draw a conclusion based on what he saw and what he heard with his own eyes and ears. Grace could say that she happened to be near the orphanage when there was an attack earlier, but he didn't want to buy the excuse so easily.