(Time skip 5 years)
Hiruzen taught Kageyama every type of Ninjutsu and Kageyama mastered them to perfection mimicking Hiruzen, he also allowed him to sign a contract with monkeys and he got himself a little one which he named Goro. His dedication to Ninjutsu shocked Hiruzen, and so did his missions, he was promoted to the rank of Jonin however, no one in Konoha knew what his relation was to Hiruzen, simply because Hiruzen did not want anyone to know that he had a student from the Uchiha faction. Kageyama also had no issues with it, he wanted to keep his hands out of the political dirt. Kageyama had now truly achieved a role of an elite Jonin and he worked in a two man pair alongside Sakumo Hatake. During this period the trio of Sanins became the official students of Hiruzen and Hiruzen went busy teaching them. Kageyama during this period retired from his role of Jonin and joined the Uchiha police faction and took his duty at the Hospital policing where his mother was also stationed.
During this period, he came in contact with many types of Shinobi, ranging from Genin to elite Jonins and even Anbu corps, during his breaks, he began reading details about their injury and how the forensic examiners described it. He got to know more about the human anatomy and slowly developed a hobby around learning it, also seeing the forensic examiners, he closely learnt their ways of doing things and had ideas to cover them. Getting a new passage of improvement in his way of Ninja, he was instantly intrigued. He was a heavy battling ninja where a heavy use of Ninjutsu and Taijutsu were used, however this method was very chakra intensive and was tracable back to him and he wished to change it.
As he began working on his methods to cover it, he researched some methods and soon saw ways of completely dissolving a corpse and relevelling the field where a battle took place, ways to cover scents and signs of Chakra release. The most effective method of dissolving a corpse quickly was a corpse dissolving liquid, however this substance was very costly and required special containers. This was a risky substance as it could even dissolve organs I around if the containers were shattered. The other equally good method were bugs used by some Aburame clan methods, raised to dissolve meat of Shinobis. However, he did'nt have access to something like that. In his research, he found a similar type of bird which could theoretically work like bugs if genetically tailored to do so. Hawks were such birds.
If Hawks were raised from a chick, fed meat of Shinobis, slowly they would begin adopting to digest the meat. Their future generations would be even better at it and so on. Also, if their diet would include some special medicines this process could be accelerated. These were the same medicines used by the Aburame and these were outsourced from the very hospital Kageyama worked in. So he got a batch of medicines for himself and slowly began experimenting on Hawks. This way, the hawks grew habituated with Chakra and developed special acids which helped them quickly eat and digest Shinobi meat. These sect of Hawks were large in size, nearly 7 ft tall, he also developed certain Hawk types with better flying speed, sharper smelling abilities and better sight, again the Aburame medicines for bugs.
This way, he created his own summons and made several of their nests in the nearby forest and created his new summoning contract. Over the next year, he continued developing more of such Hawks and began practicing with them, reducing the time it too for them to dissolve meat and for them to scout.
(Timeskip 1 year)
Hiruzen noticed this new Hawk combo techniques Kageyama used and was impressed, now he could easily track down reducing the manpower for a Ninja scouter and he also noticed manier times, their were no bodies left of the Shinobis, it was as if the bodies disappeared. Kageyama rarely went on missions as he used to be busy with his Hospital duties and the times on which he took missions, he used to flex his new skills however, he still felt that he lacked somewhere, he did not really cover all of his tracks and even after many tries, he still failed in that. So he began developing further.