Here is the question i would like you to ask...
If you had brought in ceo vaccinated birds and quarantined them for thrity days, is it still possible after you mixed the birds with the general population that they could have shed virus at the next weather change, feed change, molt, 'point of stress' etc? and then two weeks later (incubation period) you would have seen these symptoms? Had your original birds falling over dead? Since somewhere in this thread I read they were rescue layers (a la english battery rescues)/ retired production birds it is likely they have been given that type of vaccine as part of the producers normal protocol for his house.
Keep us posted (tee hee)