in OES Linux SYS is a plain NCP volume (as opposed to a NSS volume). consequently, there's no corresponding pool object and it doesn't show up in NSSMU. so everything's fine as for this.
if you remove a NCP server object from the tree it's volume objects will go "unknown" as the "volume" class has "host server" as a mandatory attribute. if you remove / replace the volume object you'll invalidate things which reference the volume object's id, such as home directory assignments. please think of this before proceeding as there are (pretty complicated) ways to "repair" an unknown volume object by adding the missing mandatory attribute and mutating the "unknown" back to the volume class.
if you don't care about all this and really agree to delete and readd the object:
check that NDS is syncing properly, delete the unknown object manually and trigger the F4 show afterwards.