Simply leaving the tacho card in the Vehicle Unit (VU) overnight isn’t an offence in itself. In fact most European enforcement agencies require you to record daily and often weekly rest periods on the driver card. To do that you would have to leave it in the VU, or make manual entries at the time of next insertion.
If you mistakenly left the card in overnight was the record made something other than rest? Strictly speaking that is an offence but most officers would call you something along the lines of ‘Numpty,’ and that is as far as it would go. Failing to put start and end locations on the record if the card is left in overnight is again an offence, again, most would ignore it in the case of a genuine error.
I wouldn’t loose much sleep over this one. Don’t think written warnings are allowed to stay on your record for very long (six months springs to mind) but others will know better than me. Did your employer provide suitable training for you on the use of the VU and your digital card?