If I sell something that is not on the menu, where does it turn up in my numbers?

On the sale, on the receipt and in the takings like any other line. The Not on the menu button on the selling screen takes a short name, a price and a quantity, and the sale carries that line. It touches no stock, because there is no dish to count down. It shows on the Z report inside the shift's sales and in Reports → Shifts and the POS. Type the same name each time, for example Bottled water, so the lines stay tidy when you read the CSV later. The POS and the counter tab in the docs covers hand typed lines.