I saw my maid talking to some men under our HDB block. It was rather uncomfortable for me to watch. Do I say anything to her or ask questions? Or do I let it be.

You can definitely ask her, but in a casual and neutral manner. Refrain from any accusatory or judgemental tones before confirming the facts. For example, instead of pointedly asking her about the men she talked with, you can say ask her if has any new friends and gently manouver her to the topic. She might tell you on her own if she isn't feeling accused...

Why is your helper not allowed to talk to men?! Do you always find it uncomfortable watching people of the opposite sex talk to each other?