Very brazen. Someone happened to stumble across the 'right' backpack, take it down from the OH bins, open the bag and locate the wallet, then open wallet and remove cash. This would have taken some time if they were being opportunistic going through 1 bag at a time unless they were targeting you and knew the bag they were looking for.

I'm curious if you were being watched handling cash in the terminal or on the flight before packing your wallet and stowing your bag.