The dealer connects the car to a power support supply during servicing, whilst it is connected to VIDA. Instructions in VIDA are explicit about the support supply not being connected directly to the car battery negative terminal as and for the reasons previously mentioned in this thread.
Nevertheless, that didn't prevent the Volvo tech who carried out my last dealer service ignoring the instruction as was apparent from my dashcam recording. It wouldn't surprise me if yours was the same.
But as VIDA also explains, the BMS system will resynch after 6-8 hours as long as the car is left parked with ignition key removed and all doors locked throughout that time.
If you have start/stop, the car maintains the battery at around 80% state of charge, so if you switch on systems such as the radio/nav unit without starting the engine you are already starting from a partially depleted battery state, and depending on the entertainment system installed it can be quite power hungry. If you have the ignition on as well, with sound on, DRLs and/or light switch on auto etc, you are looking at a current draw of 10 Amps or more.
If I plug my car into a diagnostic reader, which requires the ignition to be on, and don't connect it to a support power supply, I can also get a low battery warning very quickly. Note though that the warning probably relates to the sensed real time battery voltage, which will be lower with the ignition on or the radio/nav or other consumers running giving a false reading due to the power draw. The voltage will rise when the power draw is switched off, which represents a more accurate determination of the battery condition.
