Well, the person could be on a psychiatric hold, being held for the feds, being held for another state on warrants, being held for another county in MS on warrants, being held for prbation violations, being held for parole authorities, or being held on an investigative hold.
Sometimes a local bail bonding agency can offer you useful information on a person's status, or call the jail and see if the booking section can offer you any details.
People aren't held without reason these days, or incommunicado.