In Casper Proof-of-Stake, a validator needs to be a bonded node: it puts up a security deposit that can be lost if the validator misbehaves.
When a bonded node misbehaves, who receives the lost ETH?
From a talk given by Vlad Zamfir May 2016, lost bets and deposits will go to nobody. The reason is that if they were going to some accounts, this opens up attack vectors where attackers could control such accounts: an attacker could misbehave, "lose" their deposits, but pick up the deposits again.
ETH is never getting lost, it will be associated to the
account/contract (address) as long as the Ethereum Blockchain exists.
For the people who lose the private-key/password will not have access to it.
If my answer is not enough, means i don't understand the bonded node part.