Originally Posted by kohswisscheese

Won't I still have to go to the secure area after clearing customs? --> No, you would not clear customs if you remain airside/secure. You will exit the plane into the secure area where the departing passengers also are. You would not proceed to immigration and customs in this case.

If I have a bag in the scenario you described, what will happen to my suitcase if I don't leave airside to go and retrieve it? --> The agent will likely store it at the baggage office, not unlike what happens to bags on the carousel that are unclaimed in the US. To claim your bags, you would need to talk to an agent. There is certainly a risk that the bag will be taken by someone else, but no higher risk than if you are at the belt. For greater assurance, the baggage claim area in SIN is not publicly accessible -- i.e. it's after immigration but before customs.

Changi says the United counters open 3 hours before departure.