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.