bedok wrote:well are there actually many people that come to singapore with different passports but were born in singapore?
Plenty. At the risk of sounding like a broken record, Singapore does not grant citizenship by birth in Singapore alone.
At least one parent must be Singaporean. So there are a lot of people with foreign passports but Singapore as their place of birth because both their parents are not Singaporean. There are also a lot of Singaporeans with a foreign country as their place of birth.
Place of birth is probably not as big a warning sign (though it might be) as citizenship or PR, even if renounced. They know a lot about you if you were once a citizen or PR. With biometric passports, it is probably all automatic and no one has to be consciously alerted. The officer just has to scan the passport whichever country it is from and if any features match a former citizen the flags go up, whether you have changed your name, citizenship, or maybe even gender. Things like fingerprints and facial recognition will give anyone away.
The possibility of advance information is even scarier, because it means defaulters can be caught in transit even if they never try to clear customs.