Place them in some different locations, there's no need for passwords.
Do they contain the whole key? If yes: What if someone gets access to one of the flash drives? If no: What if one gets corrupted/stolen/lost in a fire, etc...?
If yes: The wallet should be encrypted with a strong password, which makes brute-forcing highly improbable.
If no: He could use m-of-n multisig address, and put all those backups in different places so that it is highly unlikely to have a few of them destroyed at the same time.