The sword that Harry fought the basilisk with in CoS?
It's in DD office hanging on the wall.
Re the unknown horcruxes:
We probably haven't been clued in about what types of articles will be associated with the original founders
of Hogwartz. Yes, we know about the Gryffendor sword.
But there may be other items as well. OTOH, what about
that sword? Just because it's hanging on the wall with
all the portraits and the Sorting Hat doesn't mean that
it must be free of any "contamination." Look what
Tom Riddle (Lord Voldemort) used to house a part of
his soul...a diary of his own making...the contents of which Harry destroyed with that sword (the diary
remained just a book). Was the diary a horcrux?
Doubledore worked it out that there were 7 horcruxes.
It would seem that R.A.B. already absconded with the
contents of the necklace. The interesting question is
how will Lord Voldemort respond to that message? And
how is Harry going to use that information, which HE
ALONE has now that Dumbledore is gone.