I don't think it is based on their "own discreation" rather its based on forum rules.
I see few locked threads that there's a note/post from the mod but not as always. So yeah, it will be good to see a required note to add if the mod is going to lock a thread to avoid confusion of the OP.
Exactly mods do not act base on what they think about that thing rather they take action base on the rules of the forum, most threads are mostly locked due to spamming although there are others for some other reasons but to me the number for spam locked thread are much. And judging by that once you go through a thread that is locked you will definitely see clear reasons why it is locked.
But this doesn’t mean it won’t be a good addition that a note for the thread getting locked is stated at least it guides more members again on why they get their thread Locked. Those who will like to improve their posting habit will improve. But this will also give the mod an extra work