You will not lose grafts at 3 months if they were transplanted at the correct depth. If the incision was too shallow, there is a possibility the graft has broken free but this would be extremely rare. It could be stubborn dead tissue attached to the scalp and unable to shed, although unlikely it would survive 3 months of hair washing.
It could be that you scratched your head, a spot or ingrown hair had formed resulting in scab like tissue and the graft there has shed within its growth cycle.
Was there any bleeding? Is the tissue dry and crusty?
A picture of the area you think it was located would be helpful.
This picture illustrates shed grafts and dead tissue.