Investigating a disappearance was by no means something that could uncover the truth easily.
Although the Original Work never specified exactly how long Orochimaru had concealed his actions before the Anbu under the Third Hokage discovered the slightest hint of something amiss, judging from the current situation, Shōji believed it could not have been less than one or two months. Even if the Anbu specialized in such matters, Orochimaru's methods were surely beyond the reach of ordinary people. Shōji even felt that if Orochimaru had not been too brazen, repeatedly abducting too many Konoha Ninja over that later period, the Third Hokage might not have caught his trail at all.
After all, in a country or a ninja village, the occasional disappearance was perfectly normal. Ninja going missing was not without precedent either. Spies from enemy nations sometimes targeted such people in order to gather intelligence. Konoha had encountered similar incidents over the years, so the Third Hokage had initially concluded that it was the work of enemy forces. He truly did not want to believe it had been done by one of their own, let alone Orochimaru, the disciple he had once valued most.
Some spies were successfully captured.
Others remained hidden all along, even eventually fleeing back to their own countries.
If even they could do that,
then if Orochimaru had been a little more cautious,
if the number and frequency of his abductions had not been so high, the village might never have discovered the actions of one of the Konoha Sannin.
This worked both ways.
Therefore, while the Anbu conducted their carpet search, they would inevitably seize upon some faint trace Orochimaru had exposed. What Shōji needed to do was find solid evidence before the other squads did. He knew Orochimaru was responsible, but proving it