Here's what could have pushed the unemployment rate to 3.5%

Even underemployment edged up.

According to Nomura, Hong Kong unemployment rate rose from 3.4% in December 2012–February 2013 to 3.5% in January-March 2013. HK underemployment rate also rose from 1.5% to 1.6% over the same period. 

Citing a government press release, Nomura said that the increase in unemployment was mainly observed in cleaning and similar activities, and the accommodation and food services sector while the underemployment rate increase can be attributed to postal and courier activities and the construction sector.

"Total employment rose by 8,200 to 3,720,200 persons for the period while the overall labour force increased by 17,600 to 3,852,700," Nomura said.