Why an unknown record?
If you have used your wallet in a newer version of Bitcoin Core and then downgraded, you may see this message as sometimes new versions of Bitcoin Core will write new records. If your wallet is particularly old and has been upgraded over time, it is possible that your wallet contains records that are no longer being used and have since been removed from the codebase.
It is also possible that your wallet was somehow corrupted, but if so, it is unlikely that the wallet would be opened at all.
Would that one be the HD seed?
No. HD seeds are not unknown records.
Any reason to worry?
Generally no. In some versions, the wallet will perform a silent upgrade of various wallet data which can involve writing new records. These are designed to be backwards compatible so that such upgraded wallets can still be used in older software without issue. Other upgrades that break compatibility must be done explicitly by the user via the
upgradewallet RPC. Such upgrades have downgrade protections by using either wallet flags or a new wallet version which indicate to older software that the wallet contains new critical information and should not be opened if that version does not know about those versions.
Old records are occasionally deprecated and are no longer used by the software. These are typically added to a list of known but unused records to avoid this warning, but it is possible that this was forgotten for some.