Low Oxygen Saturation without visible cause.. immediate post op.

A 55 year old Female patient was posted for Lap Cholecystectomy.
She was healthy 72 kg. Slightly over weight. No known problem with Diabetes, Hypertension or bronchial asthma. She was hypothyroid which was controlled with 5 years. Her ASA status was ASA-I with clear airway and Mallampaty score I
                           She was premedicated with Midazolam 3 mg and Pentazocine 30 mg.The  induction of anaesthesia  that was done with Atropine, Propofol, Scholine sequence with easy intubation.
Anaesthesia was maintained with N2O :O2 at 66:33 ratio  Vecuronium and Isoflorane 0.8-1 %.
There was mild fluctuation of blood pressure during gas insufflation . The operation was uneventful duration of operation was less than 30 mins . At the end of surgery and when some resp effort was noticed she was revered with usual 2,5 mg prostigmine and 1,2mg atropine.
She was extubated  after suction of mouth and airway. With in 10 mins she was alert and responding. Her breathing resp effort was normal with good forceful breathing on command. After some Oxygen inhalation 100% with mask for 2-3 mins she was sent to recovery room, where she was given Oxxygen by mask she appeared to be comfortable with no indication of pain or discomfort in abdomen / operated area.
                        After some time her Oxygen saturation slowly came down to 91-95 % in spite of oxygenation and her flow was increased to 6 litres. She was propped up in bed to 45 degree. She appeared to be alert  not in  distress or sedated. On command she could increase the tidal volume and obey command with respect to movement of limbs and over the next 20-30 mins her SpO2 remained unchanged at around 90~92% with Oxygen supplementation.
On enquiry she revealed no discomfort or distress.  Resp rate was around 18~ 22/min with good tidal volume. She said she is more comfortable in sitting up position with head up tilt of more than 60 degree and she often sat up in bed.  This indicated some airway obstruction. She neither had any broncho spasm wheeze or stridor. An injection of hydro cortisone 100 mg was given I V.emperically.
She was left in recovery for ne with close observation of vital parametres.
                    About 1 hrs later  she suddenly had a bout of  vigorous cough  and a thick sputum block was coughed out. Lo her Oxygen saturation was back to 100 % in minutes.
The sptum that was causing if any obstruction was not detectable by clinical means  with auscultation , nor the patient could feel the sputum causing any discomfort.
                    At the end of the day I will  feel like blaming injection atropine that makes the sputum thick  and can make a disaster at times like this.