Car stereo installers have gotten quite creative at building false floors or dividers. If you have a false seat back or false floor (in the trunk) covered with the appropriate automotive carpet almost no one is going to notice unless they are looking for it.

You can get a fair amount of stuff in a 4-6" false floor or a 12-15" false seat back.

You are right.
This might be helpful for anyone who is thinking about it.