Unfortunately, after the equipment was installed and plugged into the mains, black smoke came out of the back, which in my opinion is an indication of a failure of some sort.

indication 是动词 indicate（表明、显示）的名词形式，由动词词根 indicat- 加上名词后缀 -ion 构成，表示"迹象、征兆"的意思。这里指黑烟是设备出现故障的外部表现信号。