I was actually wondering if the reason would have been a suspect package, a left luggage, a note saying "there is a bomb in the plane" , a phone call stating the same... or something else.
And the winner is ....
A cool bag was left under the last seat row and found by the bomb squad.
The B737 was ex n/s and was sealed with the bag inside the cabin
The cleanning team was not founding the bag after the last flight
The BA cabin crew found the bag and declare the situation to the security staff.
The salad inside the cooling bag was not fresh
When the bag was left is another subject
