I’m not sure I agree that modern batteries shouldn’t go flat or at least, when installed in a modern car.
Modern cars consume battery in all sorts of ways when parked. Some of it’s obvious, such as the security system, radios for the remote. But some is less obvious, cars can get up to all sorts of battery draining shenanigans when you’re back’s turned.
One example is my Volvo. About 15 minutes after you lock it and walk away, there’s a clunk as the petrol cap locks itself - for some reason, that’s delayed. About 45 minutes after you lock it and walk away, the interior fans spring to life for a while, apparently it’s to help freshen or dry out up the air con ducting.
The volvo even has an electric vacuum pump as back up for the normal manifold vacuum, to provide brake assistance. The (good) idea being that the brakes should still work if the car rolls down the hill, even without a key in the lock. That circuit is permanently active, has to be, else it couldn’t be relied upon.
I’ve heard that some modern cars even have a dealer-resettable ‘deep sleep’ mode that needs to be set at the factory before shipping, else the battery is at risk just during shipping and storage.
Most 1970s cars like my Mk III cortina had none off that. Ignition ‘off’ meant ‘off’, no battery drain at all.