Problem with mystery light

["Lapitan,Edwin" <Edwin.Lapitan@xxxxxxxxxx>]

Need your help in identify a mystery light that appeared in a friend's
dashboard (e28 528e).  It's a green light that looks like this "<- ->" (2
arrows facing opposite directions, left and right).  If I remember
correctly, it's the first light from the left on the row of indicator
lights.  Thing is, it only appeared after we replaced the the brake disk and
brake pads.
 
Although it doesn't seem critical (it hasn't stopped the car from working --
knock on wood),
we'd like to know what the light stands for.  It may indicate a problem
that's not readily apparent but still important (life threatening).

That's it.  Thanks for all the help!

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