under cover of sth 是固定搭配，其中不用冠词。主要用法如下：

1. 在某事物掩护下，以某事物为掩护

They moved about under cover of darkness. 在夜幕的掩护下他们四处活动。

the yacht made landfall under cover of darkness.  游艇在夜幕的掩护下靠岸了。

We attacked at night, under cover of darkness. 我们在夜幕的掩护下于晚上发动进攻。

Moran watched every move under cover of reading the newspaper. 莫兰以看报纸为掩护，观察着每一个动作。

2. 以某事物为藉口，藉某事物之名

under cover of friendship 藉友谊之名

crimes committed under cover of patriotism 打着爱国的旗号犯下的罪行