Does leaving the car unlocked drain the 12V battery?

[hsdejong]

I've gone on vacation for 6 weeks many times and come back to my CX and it started right up, no problems.

The math:
battery 65Ah
average current consumption 60mA
discharge time is 65Ah divided by 0,06A = 1083 hours = 6 1/2 weeks

You were lucky with half a week capacity left in your battery

Variable is of course whether the battery was fully charged and still had full capacity and the continuous current consumption of the car, which may be lower than the mentioned 60mA.