Maybe, you waited for a chance during the transportation of the orb from Sachertorte's mansion... to plant a transmitter inside a part of the orb's pedestal...

The orb itself is no doubt the real thing, so Arsenius didn't notice that.

Things would roughly go like this. First, the cameras would witness the scene of the theft...

Once Arsenius has escaped, you'd follow the transmitter signal to find out where they are hiding...

You would kill Arsenius, and recover the orb in secret.

While the others are frantically searching for Arsenius, you would be carefully studying the orb in secret...