In reply to afx22:
In the car is the obvious answer.
My last couple of cars, I could get a bike in the boot (taking the wheels and seat post off) and not have to put the seats down, so no clue it was there. Current car, I need to put 1 seat down to get the forks in, but it's got tinted rear windows, so a dark rug over it helps hide it.
Failing that, just take a couple of large locks and chain to something solid - car wheel is good, or a handy tree.