Maurice is the Station Mouse, and so he must follow The Station Mouse Handbook:
Rule 1: A Station Mouse must remain unseen.
Rule 2: A Station Mouse must never go out in the daytime.
Rule 3: A Station Mouse must never approach the passengers.
Now, there's a reason why these rules exist: people do not like mice. And if Maurice breaks the rules, even to help a little boy who has lost something very important, there's going to be a price to pay...