Either with natural rest break fired by a computer lock or with suggested rest break followed by a computer lock, the only ways to know that the timer has ended are:

1. by hearing its sound;
2. by unlocking the screen and seeing if the timer has ended and, if not, how much time there is to end.

If, for any reason (ex. low volume, earphones plugged, being away at that time), the person doesn't hear the sound, he needs to unlock the screen, possibly interrupting his break if there is some time remaining.

It will be great if an option to blink keyboard's ScrollLock light can be implemented. It can be configured so that, as soon as computer is unlocked and there is any activity (keyboard, mouse), it stops blinking. Or if you can monitor for screen unlock - or even keyboard/mouse activity with screen still locked - the better.