Always ask for password when mounting encrypted partition
This is a flaw in the CoreStorage encryption system. When you first connect the physical disk, the volume is in state Locked. Once you enter the password, the volume becomes Unlocked. (you can actually see the state of your volume from diskutil list)

The problem is that CoreStorage does not lock a volume until it is physically removed from the system.

My theory is that the keys used to unlock the volume being stored within memory until the drive is ejected - although the volume may be unmounted and locked, the keys to unlock it remain within memory. Attempting to remount and unlock the partition just calls for the keys from memory instead of asking for the password.