The issue isn’t missing mileage.
The legal issue is if someone has driven under EU regs in a shift, then any subsequent off road driving is also in scope of the EU regs for that shift, so legally someone who’s driven under EU regs that shift needs to put their card in the vehicle they’re shunting in. If they haven’t driven under EU regs that shift or don’t hold licence, Digi card, then they can shunt it round fine without a card in.
The driving without card message, it’s irrelevant if you cancel or not before you put your card in, it won’t be saved to the card either way, but it will when you do a printout in that vehicle be at the bottom of the printout, as all printouts have the last 5 events on the drivers card and same for the truck