I’m not a lawyer, so I can’t give you legal advice but I’ve found a thorough answer on this site. If you find traces of a mouse in your apartment, it’s always a good idea to inform your landlord. Mice usually don’t live alone but in larger communities. If there’s a mouse somewhere, chances are great that there are more mice nearby. You can’t catch them all in the whole building, but the landlord can take the necessary measures.