No, at this point, do not book an adjustment as you do not know what it is. As kirby mentioned, finding the transaction quickly is important (if it is fraud), however, if it isn't fraud, then it is only a reconciling item.
So, on your bank rec, you have your book balance, your bank balance, your outstanding checks, and your outstanding deposits. Right now, this leaves a variance. In your reconciliation at a line item that is just called "reconciling balance" which will bring this to $0. Then, every reconciliation after than should reconcile to $0. If it doesn't, then you missed something and need to re-verify the bank statement vs the books and compare transaction by transaction. every reconciliation should reconcile to $0. Anything other than $0 (after you have reset your starting position to $0) is a mistake and you need to do it again.
Once you have established a clean process for maintaining the reconciliation without needing to change the "plug", then when you have time, you can investigate that plug to determine what is causing the variance.