I wanted to know if the key is not in the car.. can anybody open the trunk.. cause that would cause theft from anybody

The trunk can only be opened from the button on the trunk if ALL doors are unlocked (and valet switch is unlocked).
For example, when you unlock the doors with the remote, only the drivers door unlocks. In this case, the trunk cannot be opened with the button on the trunk lid. On the second press of the unlock button, all doors will unlock and allow for opening with the trunk button (unless the valet switch is locked).