The thermal mass of that mirror will allow small windows of the mirror being at ambient before the core temperature begins to release heat and create the boundary layer again. You would need several periods of cooling for that size mirror in a falling ambient temperature.
I only once saw dewing with my 18" and that is because I cooled it to 6 degrees below ambient and near morning it started to dew up. A portable hair dryer puts an end to this though and the little heat will do nothing to the mirror and quickly acclimatise back to stable mirror views. Without this you cannot really tell what the seeing actually is like. The jet stream view of fast seeing can also be imitated by core heat from a mirror.
For further information and data on mirror cooling see
Anthony Wesleys site. He has comprehensive data on mirror cooling.