I've seen police officers search for a ball when finding a baseball bat in someone's possession, implying that, without a ball, they consider it an offensive weapon.

This has been in the context of searches based on unrelated suspicions but the strong implication was there i.e "there better be a ball in the car to go with this".

Knives it's much more clear cut there. The baseball bat thing sticks out but to be honest, in the UK, a baseball bat may well be more commonly an offensive weapon than used for sport - the sport is no where near as popular.

I was going to add that I doubt officers would demand to see wickets and ball when finding a cricket bat.