When morning came, Arad was exhausted, mentally drained, and a bit depressed, to say the least. Kali didn't just drain him to the bones. He also had to deal with trying to find the mana drakainas and with the depression of failing two massive, potentially deadly creatures in his domain, no, in his very castle.
What broke Arad wasn't the failure but the fact that he checked all the maids and servants in the castle. He must've checked the two drakainas as well and failed to discover them. He had brushed against them so close yet couldn't find them.
Usually, it was natural for an adult dragon shouldn't be able to detect older dragons when they are polymorphed. But Arad was always ahead of the curse. He could even detect dragons past their twilight like Gamond.