When a comparator is measuring how filled a chest is, it outputs the corresponding power level or a signal of power level 0 if the chest is not accessible. However, it does not update when the chest gets locked/unlocked unless it gets a block or redstone update.

Expected behaviour:

The comparator should update the output signal when the container is accessible or not by the player.

Actual behaviour:

The comparator does not update the output signal unless it gets a block or redstone update.

This can be seen in the attached video.

Possible fix:

This could be easily fixed by adding a "locked" state to the chest, that would be "false" if a player can access the container, and "true" if it is not, i.e.: a solid block is on top of the chest or a cat is sitting on top of it.