Why are some so quick to accuse the pilots of doing it wrong? Who knows what happened, maybe it was as they were rotating, maybe the eye witness got it wrong as is often the case. Most aircraft have no issue flying with a door open, maybe as I would they decided not to worry about it at a critical phase, rotation, and come back for an uneventful landing. Maybe it did happen earlier and they did not notice the light, so what, people miss things sometimes, just human nature, I am sure you would not have missed it.