Even with a Roth 401(k) contribution there would be a code in box 12 (code AA). If you made contributions to your employer's 401(k) plan via salary deferrals, the employer must report them in box 12.
Are you sure that the employer has a 401(k) plan to which you were contributing. Some employer's allow a portion of your paycheck to be directly deposited into an IRA instead of to a checking or savings account, but that would not be an employer plan.
