If it was handed in or found only today, it will be at the library's lost property which you can ask for at the Library Reception desk.
If it was handed in or found yesterday, it will be taken to the University Security office Lost Property at the end of the day.
You can check whether it was handed to security by talking to a security officer at the University Reception desk.
You can contact the University Security by phone:
Cambridge Campus: 01223 698368
Chelmsford Campus: 01245 683628.