Be real
We are talking about skylines here, not Mclarens or Enzo Ferraris. If I was inclined to go out and steel a line I could find 10 parked in any shopping centre, and several parked in and around any business district.
In addition any of us with insurance have had to fit at least a 3 point immobiliser, so I would have thought there are plenty of much easier targets out there.
Shopping lists of cars are useful for everyday cars (Holden, ford Mitsubishi) because there are plenty of spare parts needed by the crash repair trade, the smallish skyline community makes the risk of butchering cars for this industry a lot riskier. You are much more likely to have your car stolen for a joyride than a shopping list.
Don’t get paranoid, just fit a decent immobiliser.