Oracle de Belline: Card 21: Theft/Loss

https://lh5.googleusercontent.com/-...I/AAAAAAAAMSY/YboBbH8T7r0/w354-h577-no/21.jpg

French Name: Vol Perte.

Number: 21.

Planet: Mercury.

Appearance: A blue, scaly bat flies with a rat in its claws. The bat is grinning madly.

Keywords: Fear, Trickery, Theft.

Personal Notes: Edmond doesn't seem to know jack sh*t about bats. They aren't scaled, they don't eat rats (most species are insectivores), and they most certainly aren't blue. The bat is an allegory for trickery and misdirection, said to be exiled by the birds and the beasts for trying to fight on both sides. The bat is also a symbol of luck in Feng Shui, and a symbol of death in Mesoamerica, but that doesn't seem relevant to the card. Rats are a symbol of theft, as seen in the Lenormand deck. A bat carrying a rat, therefore, could mean conmen and scamming.

It's a long shot, and please take it with a grain of salt, but just my thoughts anyway...
Maybe a rat is robing a rat.
Furthermore, in Greece, in the middle-ages, there was a myth that bats arose from a mouse. The myth states that a mouse came into a church and stole one of the wafers of the eucharist, and then decided to keep it rather than to eat it. Because of the mouse's pious respect for the holy wafer God gave it wings so it would be able to find plenty of food in the future, and so it became a bat. In Mexico peasant farmers used to refer to bats as 'ratones viejos' meaning 'old rats', they believed mice grew wings as they got older and turned into bats.
In were I live we use to say:
"A thief who steal from a thief is forgiven for an hundred years"
This could reinforce what you said about being about conmen and scamming.