For example, when you start the vehicle in the morning, the OBU display is showing 0 km driven and €0 toll to pay, but 10 metres down the road, the OBU display suddenly shows: 3 km (driven) for a certain amount. What’s going on?

This is actually the remaining part of the toll calculated during your previous journey which wasn’t sent by the OBU to the Satellic system and was thus still pending. The OBU couldn’t send it to the Satellic system because it went into sleep mode too soon, there was no GSM network or the OBU was still verifying the road on which the vehicle was driving by matching the GPS position with its internal map. The OBU notices the pending toll the next time you drive this vehicle and sends it to the Satellic system as soon as possible.

Be assured that you will always see the correct data on your billing documents. The toll in question was in fact registered on the right date, it was just not processed until a later date.