Each game fork has its own rules. Additional forks may be possible if the particular game would allow it at the time. Reunifications must be legal in all affected forks.
Is it hiding in a cupboard in the low-fat cherry yoghurt pie factory in the northern suburbs of Boston? Forgive my obsessive interest in pies; I'm just gathering data to put on a chart.
[lurker] You seem to have a very good grasp of it then. I meant, of course, the cherrywood cupboard next to the door in the emulsification quality evaluation department on the 3rd floor.