under the cover of darkness ------- 在夜幕的掩护下（成语）

英文释义
(IDIOM) Describing something done secretly at night.

例句
Last night a group of thieves broke into our factory under the cover of darkness and stole a lot of valuable equipment.
昨天晚上，一群窃贼在夜幕的掩护下溜进了我们的工厂，偷走了大量的贵重设备。