private void throwIfStaticMemberIsUnknown(ConsumerGroupMember staticMember, String receivedInstanceId) {
        if (staticMember == null) {
            throw Errors.UNKNOWN_MEMBER_ID.exception("Instance id " + receivedInstanceId + " is unknown.");
        }
    }
Validates if the received instanceId has been released from the group

@param staticMember          The static member in the group.
@param receivedInstanceId    The instance id received in the request.

@throws UnknownMemberIdException if no static member exists in the group against the provided instance id.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java