I would imagine they are looking for Cigarettes, as those are heavily taxed in Singapore. I actually now recall being scanned on a flight from LAX, but that was just random, not the whole plane.

I had a large suitcase filled with Cereal, and various mexican food products which are hard to find in Singapore, or stupidly expensive, they didn't even ask to look inside, even though it must have looked odd on the x-ray. They said just looking for Cigarettes and Alcohol