Hades had stored the demon's diary in a treasury room in his new temporary palace. It was supposed to be an impregnable defensive area guarded by 16 tier 4 demon generals , however he had gravely miscalculated on the amount of required security.
There were many runes that detected demonic presence in the area, but none for humans as while it was possible to enter Hades's private space through hell , it was not possible to do so from middle earth unless Hades himself invited you explicitly like Rudra was.
Since humans were not allowed in hell at all , the security system was not tuned to detecting human presence.
Dronacharya hence managed to luck out when it came to the first layer of protection from entering the room which was runic inscriptions that detected demon intrusion.
