Red light on left corner of mi2s

Oct 4, 2013
Hi fellow users,
As far as i know there is only 1 notification light on the xiaomi 2s which is the one below the home button.
Today i noticed a red light on the top left of the phone while i was on airplane mode, beside the proximity/light sensor.

After a while it turned off itself. I was unable to recreate the incident. Anyone knows what that red light is for?