Spoiler
I don't know if this is exactly how you are supposed to do it, but it is how I did it. I clicked on the rat, and it escaped to somewhere else on the building, the direction you see it run has no relation to it, the location is almost random. After doing that a few times it appeared at the end of the hallway to the right of the reception desk. It dissapeared all the same, but then I put the snacks I got from the vending machine on the spot the rat was standing at before dissappearing, and the rat returned to eat them. Then I used the jar on it and caught it.Reading other people, maybe I’m misremembering something, because they talk about setting both the snacks and the jar as the trap, and I only remember setting up the snacks.