Heh, I also have an NC2 Miata, and I don't drive it for 3-6 month periods, and it still starts on the first/second engine spin.
If the battery is over 3 years old, I would replace it and start anew simply because of the life/abuse it'd had so far.
And then I'd start at the obvious ones - radio, aftermarket radio, alarm, etc. You can literally put a multimeter along the fusebox to see what is pulling voltage after - but most likely it will be something along 12V connected switch.
Wiring isn't problematic, but if you have a hitch installed it may be wired to always be on.
If the battery is over 3 years old, I would replace it and start anew simply because of the life/abuse it'd had so far.
And then I'd start at the obvious ones - radio, aftermarket radio, alarm, etc. You can literally put a multimeter along the fusebox to see what is pulling voltage after - but most likely it will be something along 12V connected switch.
Wiring isn't problematic, but if you have a hitch installed it may be wired to always be on.