I've transited Hong Kong, Tokyo (NRT), Singapore, and Seoul (both SEL and ICN) without going through immigration or customs. All of them have a security check.

I've transited Hong Kong, Tokyo (NRT), Singapore, and Seoul (both SEL and ICN) without going through immigration or customs. All of them have a security check. They look at your passport and boarding pass, and x-ray carry-ons. But it's not full immigration.