Old DNS server still in records

This is probably an easy one...

When I do a `dig ns mydomain.com` it returns mostly correct records, but
includes a server that no longer exists (lets call it oldns.mydomain.com).
At the registrar where 'mydomain.com' is registered, I do not have
'oldns.mydomain.com' in the list of registered name servers (the ones that
should be there, are). How do I find out where 'oldns.mydomain.com' is
still registered as a nameserver, so I can remove it?

Hope that makes sense.

TIA