src/Store/IdentityMap.php
public function exists(ResourceIdentifierInterface $identifier)
    {
        $record = $this->lookup($identifier);

        return is_object($record) ? true : $record;
    }
public function exists(ResourceIdentifierInterface $identifier)
    {
        $record = $this->lookup($identifier);

        return is_object($record) ? true : $record;
    }
Does the identity map know that ths supplied identifier exists?

@param ResourceIdentifierInterface $identifier
@return bool|null
the answer, or null if the identity map does not know