In this day and age of immobilizers, are they really looking to steal the car?
After getting my car broken into many years ago (nothing was stolen as you aren't meant to leave anything in your car), I never locked my car again when at home - open up glove box, open up centre console, leave car unlocked.
assuming you do not run the risk of having a bum sleeping in your car, this is the best option - doubtful they could get the car started (and if they could / were looking to do this, would they really not break a window to get in)
it wasn't until I had $5K of damage done to my car (bonnet, roof, interior - why couldn't they just enter through the side window - well, bc they were idiots, that's why and figured side window breakage would set off factory alarm yet sunroof wouldn't?) that I really thought about this problem and came to the above conclusion