Err... just a few thoughts I had:
So in the scenario as in the topic title,
we should just be honest and tell the interviewer "Yes
i am currently working somewhere else" ...?
But then he'll sure ask "Why you wanna leave
the current job", right? Haiz.
but anyway just don't badmouth your current company in any way
lah.
Oh, and probably don't ever say "I took MC to come here
interview" is a good idea 
say you took AL or timeoff or something.
Finally... interviewer won't mind that im "trying to jump
ship from another company", will he...? I guess sgporeans must
be all quite realistic/practical abt these things, huh...
Say you want to try a different environment. It's part of your
personal life adventure.
