Well now that I've been mentioned, my wife's 2015 VW Polo 1.2TSI 100PS, has only ever had its battery dongle lead removed once, and that was by a forgetful VW tech when replacing the gearbox upper mounting rubber.
Edit:- how did I work out that that lead might be disconnected, well since it had come back from that warranty visit, auto Stop/Start had been inhibited, so it was easy to put 1 + 1 and get straight to the cause/reason why!
Currently my wife's Polo has been exhibiting an annoying "thing" wrt to charging!
Out of curiosity maybe, I bought a plug in DVM that has a connector that allows it to get plugged directly into the "ciggy lighter" socket, prior to that, probably when the car was new, I did connect up a DMM and so that I could see what was happening when wrt voltage at that point - ie lowish voltage of 12.5>12.7V while under power and 13.5>13.7V when on the overrun.
For the last 18 months or so, until 2.5 weeks ago, that was still the case, a quick glance down at that DVM every few minutes to make sure all was still okay.
Then, one day about 2.5 weeks ago it changed, 14.7>14.9V constantly, when cold under power, when hot under power and on the overrun at cold or hot, which meant that the battery was ending the day proper stuffed full of charge - I was not too happy about that.
I checked the battery stats as well as checking for logged faults - no faults logged, the battery stats claimed all was well and the voltage was what was required, I stuck with this for a few days, trying to work out what to do next, new or used alternator to fit until I had got the original one checked over, a problem with the battery management part of the CAN Gateway controller, an out of spec battery monitor unit, or a dying battery. The battery stats proved that the car's system thought that the battery had decayed in useful capacity from 59Ah to 34Ah although I could not find where the "energy throughput" statement was - that I believe is where the battery usage gets counted and when it reaches a certain value relative to one of the theoretical battery design specs, it is considered ready for the dump.
Edit:- I even resorted to charging the battery overnight to see what effect that might have on this constant high charging voltage - no change!
As one last idea, I changed the battery serial number by one unit, then used the car the next day, for the first 20 minutes it remained as before, then it changed to doing things the way I had witnessed for its almost entire life with us. So after maybe 200 miles and maybe 6 outings including a 24 miles each way journey, it is back to being almost as before, at first the battery was being run well down before charging happened on the overrun, but now it has crept back up to roughly 13,2V under power and roughly 14.7V on the overrun, so not exactly back to where I had previously expected it to be.
I did check that auto Stop/Start did operate when this first started happening just in case the battery was now not fit to be kept up to that charge level - but it always shut down at lights etc.
My next step, well, early on, probably driven by tales of early life failure of EFB batteries, I ordered in the next size up battery tray and jacket - and now I've bought the next size up Bosch AGM battery, talking online to someone that fixes these cars, he said that just too many battery monitor units have been failing, so just in case that this is an out of spec battery monitor issue that will attempt to kill my new battery, I've scatter cashed this and bought in a new battery monitor unit.
My, baseless thoughts on this are, that one possibility is, these battery monitors can misbehave and the owner nothing about it until the battery dies - I'm sure that less than 1% of people are bothering to keep an eye on the voltage or the charging behaviour.
I was told that for instance, VW Caddy battery monitors are on back order currently and they end up on back order frequently, now maybe only VW Caddy battery monitors are not "too good" - I don't know, the battery monitor on that 2015 VW Polo is made by Lear Corporation in Spain, and maybe "annoyingly" for me, when I got one ordered in, they were in stock at the warehouse and the new one's Lear manufacture date was 07/08/2020 and the VW Group stocking date was 14/07/2021 - so maybe not demand wise in the same league as the VW Caddy version, who knows, as the cost of the battery monitor is roughly 1/3 of the AGM battery I bought, I have considered, in the absence of any better or firm ideas or worked out facts, either I hand it in and it seems that from an outsiders view of this NFF and pay for that report, or I use that money spend on parts!
Sorry that this does probably not help out the OP - or does it give some insight maybe, maybe buy one of these cheap (£5) plug in DVMs - when I mentioned this issue while trying to help someone on another forum, the first reply to my posting suggested that my cheap DVM was the issue, that would have been just too good to be true, but I had checked that out in the first place.