They are definitely tracked, in order for them to gerrymander
the election boundaries.
Why do you think the election boundaries look the way they do
now? 
If you don't think that they track your votes, you should go
with your group of friends and exchange each other's ballot paper.
See if they stop you from exchanging. I once went with my parents,
tried exchanging the ballot slip and was stopped.
If all ballot slips are the same and unmarked, there is no
difference if I switch around the ballot slips, unless they are
tracked by some invisible serial numbers.
During the last election, Ling How Doong (opposition candidate)
did claim that the vote counting officials took a long time to
place all the ballot paper under a "lamp", they could have
improvised a scanner under the lamp. There is no need to break open
the sealed ballot boxes after elections to count the votes
again.
Even if they have knowledge that you vote for opposition party,
there is nothing much they can do, unless you are a
high ranking civil servant (your promotions and length of
service will be affected).