obviously something is drawing down on the power. So yeah check all ancillary gear (interior lights etc..) and earthing straps.
I've experienced and heard a lot of people with R32 GTR's who cant leave their GTR sitting for a week or longer period of time. Some of the blame is the car alarm drawing down on the battery.
I did have the several small watch type batterys powering the m80 mongoose alarm drain after several years and had the siren part replaced.
(after ripping out the alarm and smashing it as it makes that orrible dull sound when going flat ....)
Buy a charger and put it on trickle charge whilst not driving it often.
Because if you just disconnect a battery lead terminal and if you have an alarm/imobiliser it will trigger the alarm !
The other thing I have found is that the spark plugs foul easily if it is idled for a while whilst stationary and then turned off, which makes it harder to start next time and drains the battery further while trying to crank over.
I like mine to start every time and on first turn of the key rather than 2-3 cranks. I drive it every other weekend so not often.
So replaced battery, alternator, starter, plugs', ignitor, fuel pump & filter
I understand that the OEM ECU detects the location of all cylinders on cranking. Some of the aftermarket ones look for the position of a particular piston (#1) being TDC so crank a few times on start-up
If your battery is tired this can be an issue.