Reconciliation Discrepancy expense is posted when you close a reconciliation that is out of balance and allow quickbooks to make an adjusting entry. An entry from 2015 should only show in a P&L that includes 2015.

The way to remove the discrepancy is to replace it with its proper accounting

That account doesn't even exist until someone forces QB to make it automatically by clicking Done on a Reconciliation that is out of balance. And the other issue is, even if you see One Entry, that could be a net mistake. Example: I see it is $15. But in the banking details compared to the Statement, I see overlooked deposits totaling $50,585 and overlooked Expenditures totaling $50,600. So, Net of $15 is not that innocuous, after all.

If it worries you, present it to the CPA. They evaluate the issues as "material or not" = if they required you to also provide bank statements, they would have noticed if this seems severe enough to straighted out, back in 2015.