This is how Boxcryptor works and not a bug. The .bc "file" is actually a bundle - a folder which appears as a file in Finder. After you entered the password, this the .bc folder is mounted to the virtual Boxcryptor Volume (/Volumes/Boxcryptor) and the path stored in Boxcryptor. When the .bc folder is mounted, no password is required to access the files until you exit Boxcryptor.
If Boxcryptor is running, the virtual Boxcryptor Volume stays mounted even if you move the underlying .bc folder - but you will receive errors when trying to access the Boxcryptor Volume because the underlying folder does not exist anymore on the stored path. As soon as you move the .bc folder back to its original location, you can access the Boxcryptor Volume again (because it's still mounted).
If you want to have to enter the password again, you have to either unmount the Boxcryptor Volume or exit Boxcryptor.