Whether or not the plan allows the funds to stay in the plan, automatically rolls the money over to an IRA or simply makes a distribution of the balance in the plan generally depends on the amount of the balance in the plan. If either a rollover to an IRA (which would be a direct rollover) or a regular distribution is made, the plan must issue a Form 1099-R. The IRS apparently received a copy of this Form 1099-R from the plan, so apparently either a rollover or regular distribution happened in this case. It's possible that lack of a proper mailing address resulted in the Form 1099-R and the funds (either a rollover or a check) going missing. If the funds were rolled over to an IRA, the IRA custodian would have issued a Form 5498 from the IRA acknowledging receipt of the rollover contribution.
You might check your Wage and Income Transcript for the year in question. It would show the Form 1099-R and, if there was a rollover to an IRA, the Form 5498. (If a rollover was initiated near the end of the year but was not completed until the following year, it would be the following year's Form 5498 that would show the rollover contribution.)
https://www.irs.gov/individuals/get-transcript
