Land of Grass.
Near the Konoha ninja forces' camp, Mu Yue was teaching Observation Haki.
Besides Obito, Shisui, and the others, Minato Namikaze was also learning Observation Haki from Mu Yue.
Mu Yue had brought it up himself. His reason was that it would be exhausting if he were the only one capable of identifying Iwagakure spies, so he needed Minato Namikaze's help.
Mu Yue knew very well that once the intelligence from the Land of Grass was sent back, no matter how highly Hiruzen Sarutobi regarded him, he would find a way to make him hand over Observation Haki.
Even if the person who possessed this Ninjutsu was not Hanyu Muyue, but Hiruzen Sarutobi's grand-disciple Minato Namikaze, or Hiruzen Sarutobi's disciple Jiraiya, the outcome would be the same.
Any Ninjutsu capable of playing such a tremendous role in war had to be handed over to Konoha, no matter who possessed it. Otherwise, they lacked the Will of Fire and did not love Konoha.
Since he would have to hand it over sooner or later, Mu Yue naturally chose to teach Minato Namikaze first, since they had a better relationship.
Mu Yue did not particularly mind Observation Haki being spread outside. After being adapted to this world, Observation Haki had a far higher threshold than Armament Haki. Perception talent alone eliminated a great many people.
And even having perception talent did not guarantee that one could learn Observation Haki, eliminating another large group.
Even those who could learn it might not master it well, filtering out yet another group. And even those who mastered it well might not possess Observation Haki's special abilities.
Konoha only had a few thousand ninja. After all those layers of selection, there would probably be only a handful of ninja over the next few years