XXL 29er plus 26er in back of my Civ (plus the XXL tall 29er rider in front comfortably, plus myself driving), plus tools, spares & luggage, but yeah that’s with seats down 😀 . Comfy drive to Alps.
Had bikes stored in back of the car when staying places without much worry. Back up the rear to a wall, cover over the visible bits of the bikes. Parcel tray hides the rest. Hard to break into, would be noisy, alarm, and unless they can get a door open it would be difficult to get them out of a window. Miles safer than my garden shed even.
Insurance is the tricky bit though. My house insurance insists must not be visible in the car. Not sure if a cover over bikes is enough. Car insurance is unlikely to pay up much if anything for contents stolen.