Maybe nothing. Rain the night before or was the radiator wet with dew on startup? Could be water boiling off the surface of the radiator. Not uncommon to see after driving in the rain.
If still coming out after sitting on a warm dry day...then it could be a leak.....but then there would likely also be dried antifreeze residue on the radiator surface near the leak.
Current Chevys -'19 Silverado RST;'18 Malibu LT Redline;'18 Traverse Premier Redline<br /><br />Previous Chevys:16 Silverado LT;16 Malibu LT2;'17 Impala;'16 MalibuLT;'15 Camaro LS;'15 Traverse; '13,'12 and '07 Avalanche LTs; '11 Tahoe LT; '13 &'11 Malibu LTs; '08 Trailblazer;'06 Colorado WT..... Chrysler era.....'80 Citation;'75 Impala;'72 Chevelle;'69 Camaro R/T;'65 Impala SS;'62 Nova