how could the mh370 fly undedected, page-9

Imagine they would be turned off while on the ground as they are not necessary at that time, and may well 'clutter up or interfere with ATC systems'

A transponder is mainly/only used to send a "signal" to ATC to IDENTIFY the particular plane, and maybe to boost signal strength to ground radar.

Whether it is on or off, the plane can still be tracked by standard ground radar, so why does nobody know where this plane is?

Still believe someone knows its location/condition, but is not sharing that info. with the rest of the world.