[jggimi]

It seems that /etc/soii.key is built by rc(8) when making keys (first boot):

        if [[ ! -f /etc/soii.key ]]; then
                openssl rand -hex 16 > /etc/soii.key &&
                    chmod 600 /etc/soii.key && sysctl -q \
                    "net.inet6.ip6.soiikey=$(</etc/soii.key)"
        fi

It can be recreated easily.