Basically when you select the option on the bindstone to take you back to your grave, you give up your grave to be teleported to its location. So if you choose that option, your grave will no longer be there.
As for the "someone," were you poisoned? If an enemy kills you ,for example, via poison, but is not in the same area as you, you will get a "someone kills you message." It is also possible that your Con buffs expired, and lowered your HP to below 1.