def check_opreturn(script):
    """
    Checks whether a given script is an OP_RETURN one.

    Warning: there should NOT be any OP_RETURN output in the UTXO set.

    :param script: The script to be checked.
    :type script: str
    :return: True if the script is an OP_RETURN, False otherwise.
    :rtype: bool
    """
    op_return_opcode = 0x6a
    return int(script[:2], 16) == op_return_opcode