Originally posted by Denholm 
You forget that icons can be turned off from the FV after you select the padlock target. 
 And I believe when you lock on to them and then turn off the icons... you lose the padlock.  I have tried this before and you just need to practice with your views and tracking when your making a film.  Hence using multiple shots from different angles or practice the movements on your keypad.
EDIT: Just tried it.  Opened film, started it, found a target with Icons turned on and hit the padlock button.  Then turned off the icons... lost the lock.