I was reading some posts here where some have left stuff in the trunk area and then closed the top.. end up ruining the motor or the roof closes crooked. Anyway, this might make sense already to some but when i first got the car a couple of years ago I also wondered why no sensor to tell you something is in the trunk?

Well, it does. Sort of.. You should never ever put anything in the trunk without that flap shelf being down. This is why there is a sign in the trunk not to put anything on there when the flap is not flat. If you follow this rule strictly, then if you ever open the top, it won't let you unless that flap shelf is snapped in place. So if you get that message on the dash to close the flap, you will have to physically check the trunk and make sure nothing is there when the top closes. It is kind of a weak solution, a real sensor would be nice.