If the PCs choose not to move on and leave whoever or whatever it is trapped in there, the unseen spirit will explain that an enemy that he or she had in life has trapped the spirit within it's dead body a few days ago, when the entombment took place. It begs the PCs to open the tomb and the coffin as only then will it be freed to pass into the nether-lands of the afterlife. It promises that the PCs can take the grave goods with the body without any spiritual comeback that often happens to those who steal Funeral Gold and Grave Silver. If the body was a noble lady in life, it will have rings, bangles and necklaces upon it-if a knight, he was buried with his magical weapon (which can be anything from The Sword of Fire in a low-magic world to a highly magical weapon in a high magic world.
If the PCs to decide to open the tomb or coffin, one of three things could happen...
Option One-The undead spirit was telling the truth. When the tomb is broken into and the coffin opened, a glowing ghost appears, thanks and blesses the PCs, and vanishes. The PCs can loot the dead body without the looted items being cursed, although it would be wise not to show them too openly in the local area, as it's still illegal grave robbery.
Option Two- The undead spirit was indeed trapped in it's tomb but for a very good reason-it's a very dangerous form of undead. When freed it attacks the PCs, perhaps even being powerful enough to open the graves around it and summon lesser undead to join in with the attack. Or it thanks them, vanishes and it is not long before the PCs find it is stalking and killing various people including an NPC that they need to keep alive for some reason.
Option Three- There is no undead spirit trapped in the grave and it is magical ventriloquism. This can be anything from a student prank by a student at the nearby magical university to a trap to get them in trouble with local law enforcement for grave robbery to a booby trapped tomb that harms or indeed kills whoever dares open it that the PCs were tricked into opening.