By the way, my friend’s cars were BMW and Mercedes.
Here’s a quote from an article four years ago in 2017.
‘…While their owners are flying off to visit friends and family, tens of thousands of automobiles will be left waiting in airport parking lots. Some of these travellers will return to discover their cars’ batteries are dead. Welcome home. Here’s a set of jumper cables.
Why? Modern cars consume electricity even when they are seemingly dormant: GPS systems, proximity sensors, antitheft systems, data links and automatic vehicle-condition “polling” all draw on the battery.’
Another thing is people wondering why their stop-start system isn’t working in their cars. Once again this is down to the car not being driven enough to top up the battery for it to keep restarting the car, so the system will deactivate until the battery has enough charge.