There is an IFTTT-Applet that turns on your phone’s bluetooth when you get home (or enter a specified GPS area).
The same applet may be created the other way round like: When you leave home (or exit a specified GPS area) turn off bluetooth.
I know not everyone likes to use “yet another app” to achieve this and of course it could be implemented by the Nuki devs into their own app, but I personally think they have more important stuff to spend their development resources on.
At least that’s my personal opinion. I think this is not providing a big benefit for a huge user base as most people leave their bluetooth turned on 100% of the time to keep their established connections with their car, headphones, smartwatch or whatever other device/gadget.