I have a saved passphrase for something named Radon from March 2015. The current wallet doesn't seem to have a passphrase backup option.
Was this for testnet or some deprecated client version? I can't remember.
It is most likely from a depreciated wallet version. The wallet.dat should be compatible with all the current and passed clients, however the blockchain is not, so a resync would be required.
The thing is I don't have anything except the passphrase. Is there any option to use it now? I want to check if there were actually any coins there.