Andrew expressed precisely :
Modern cars are far more busy even when standing parked. If you
regularly leave a car made in recent years unused for more than
a few days the battery will already have significant drain.

I'm not sure you are correct with the 'significant drain'. My last car
would gradually move its systems into normal sleep mode over the first
30 minutes. If the car remained unused for longer than 4 weeks, it
would then go into a deeper sleep mode, switching off the remote key
receiver, then you would need to use the actual key.

My present car drops down to a 20mA draw sleep mode within a couple on
minutes. My car has a 75aH battery fitted, which equates to it being
able to maintain that level of discharge for 3600 hours full to flat -
150 days, but probably much longer than that. Not that much worse than
I would expect just leaving the battery disconnected from the car and
my car has a mass of equipment on standby, so a more basic car will
likely have an even lower discharge.