Can you detect the light lost in a fiber optic through the jacket?

Hey guys,

Was just working out in the field with a handheld tester for fiber optic; and the tester was able to determine if there was bidirectional traffic flow on a fiber optic cable, simply by placing a clamp around the fiber and pressing a button.

Can anyone enlighten me on how this works or if I am just misunderstanding something.

I understand you could measure the electromagnetic field around an RF cable due to loss into the air; just wondering if this is what the fiber detector was doing.

The meter I used was a AFL Optical Fiber Indentifier - OFI-200D