Aren't MRT stations public areas? I have taken photos in many stations before and no one came to tell me anything. But I was a pns..
For airport it is a different case. I was at the arrival hall waiting for my dad to come out and I saw this cute baby girl leaning on the glass, so I took out the same pns and started to snap.
3minutes later, the officer behind the glass door, came out and asked me to stop. But what so secretive about that area? Even if I don't fish out a camera, I can use my eyes to look and see everything.
That was at terminal 3. Has anyone seen the "No photography" sign there?