Thanks for the tip. I had a look and can see some bluetooth options. I do have a decent topdon battery tester which tells me what I need to know I guess. It's only really a problem in winter.
But there definitely seems to be some extra phantom drain on the car I have now. Didn't have this problem with my previous Fords or Mazdas which would also sit unused for a week quite often and never replaced the batteries except on a MX5 which is understandable I guess.
In most modern cars, a phantom battery drain is likely due to either 1) an added third-party stereo head system, or 2) just the phantom power of the active alarm system itself while the car is locked, which also serves to prime the fuel pump when the doors are conversely unlocked.
If I have a car sit for months, I just throw a trickle charger on the battery once a month to top it off.