Yes, there is a way to configure Odoo 14 to show a warning message when additional products are scanned during an inventory transfer. You can achieve this by using a custom module that implements a validation check before the transfer is validated.

Here are the steps to implement this solution:

1. Create a custom module in Odoo using the Odoo Developer Mode.
2. In the custom module, define a new field on the stock.picking model to store the scanned product information. For example, you could create a new field named 'scanned_product_ids' of type 'many2many' to store the products scanned during the inventory transfer.
3. Create a new method on the stock.picking model that checks whether any products have been scanned and not included in the transfer. You can compare the scanned products with the products in the transfer lines to identify any discrepancies. If any additional products are found, raise a validation error with a warning message.
4. Override the 'button_validate' method on the stock.picking model to call the new method created in step 3 before validating the transfer.
5. Install and test the custom module to ensure that the warning message is displayed when additional products are scanned during the inventory transfer.