Last time I was too blur that I locked my S pass in my luggage, and so couldn't provide it at the checkpoint. I instead used my student matric card(NUS one) to identify myself loh. They brought me to another counter at the very end. It took them few minutes to check database/info, whatever lah...then they let me go with a kind reminder: "pls bring along ur pass next time!"

So as long as you have sth to identify urself in sg(other than your passport), should be ok lah. hehe...no worries. Anything can just clarify at the checkpoint, they won't send u to prison lah...

Gan3 gan3 lai2!

to be safe, try to get some certificates from the reputed/famous organizations if possible