When "Death Intuition" flickered, Arthur's heart was filled with surprise.
As "Eagle Eye" and "Insight" twinkled, confirming that Harris harbored the intent to kill him, Arthur was even more surprised.
According to his conjecture, Harris would be annoyed and unwilling, but he definitely wouldn't harbor murderous intentions.
Unless...
The young 'Spirit Medium' thought of something—
"Could this guy have secretly kept some of Isidore's gold?"
Just before the downfall of the 'Mouse Council', Roschek, seeing the unfavorable situation, had taken away the most important part of the council's wealth: Mystic Tools.
And after the downfall of the 'Mouse Council', Isidore should have taken the remaining Mystic Tools, including "Hercules' Key".
But these were only the most important parts.
There were less important parts too.
For example: real estate.
For example: gold.
The real estate could certainly not be in Rat Street, it had to be in the Shire District.