This has to  do with the Change in the Capital Gains tax calculation that the IRS sent to the software providers initially for 2018. IF you recall, when the IRS found the error, ProSeries did an update, but the IRS said to NOT AMEND those returns already filed. They had a way of knowing which returns were impacted, and would do a refund to each of them.  I have had about 4 returns do that... and 3 of those clients called me when they got a check from the IRS last summer.  When I opened those returns.. I got the 'detected a diff" (because ProSeries did the update), and each of those differences matched those clients refund. One client didn't tell me, but I asked him about it when he came in this year.. and he acknowledged the extra refund ck to last summer.  I suspect your client got a check last summer too.