While the French army under General MacMahon engaged the German 3rd Army at the Battle of Wörth, the German 1st Army under Steinmetz finished their advance west from Saarbrücken. A patrol from the German 2nd Army under Prince Friedrich Karl of Prussia spotted <answer>decoy fires</answer> close and Frossard's army farther off on a distant plateau south of the town of Spicheren, and took this as a sign of Frossard's retreat. Ignoring Moltke's plan again, both German armies attacked Frossard's French 2nd Corps, fortified between Spicheren and Forbach.
What did a German patrol spot that led them to believe that Frossard had retreated?
decoy fires