I was there to witness..
I happened to be plane spotting on that day and noticed the airport was extremely quiet during the busiest period. In fact, 4 F-16s were deployed. One on its tail, the other slightly ahead and another 2 circling the airport. They continued to circle the airport for the next 15mins after the Cessna Caravan landed. It landed on 02C and parked at Bay 404.
Apparently, they had no flight plan. It was spotted at the North Eastern part of Singapore. Also, it was heard that they were not contactable initially as the ATCs were trying to ident the aircraft flying.