It is a common mistake to think of the English meaning of the word 'contains', and believe that therefore contains? will tell you whether a vector or array contains a value. See 'some' if that is what you want.
contains? is good for checking whether a map has a mapping for a key, or a set contains an element. It can be easier to use correctly than 'get', especially if you wish to allow a key, value, or set element to be nil.