I fly in and out of Changi quite a bit, only time I have ever been scanned is in April on a Air Asia flight from HKT, they asked for our passports and searched everyone right after disembarking the plane, and then scanned everyone's luggage. I think they were looking for something or someone specific.

But all in all the process only took an extra 10-15 minutes. Still got out of the airport faster than most airports in the world.