If the holiday was booked and approved then the subsequent sickness period has no relevance as to where or why the holiday was taken. You state that the reason for the investigatory meeting was that permission was not granted. There is some disparity here with the narrative.
If the permission was granted, as I have assumed in my response that no meeting was needed under the guise of failing to ask permission.
There appears, on the meagre facts presented, no reason for dismissal.
Missed my nose again- damned spatial awareness syndrome.