Likely that the gas cap is not on properly, or that it was put on after the key was turned back on. The fault light in that case is that the evaporative emission control system (which captures gasoline vapours) thinks there is a leak.

With the key off, take the gas cap off and put it back on tightly. Then drive normally; if that was it the light will go off within a day or so.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue