If something happens silently, especially something bad, you do not notice that it is happening.

中文解释

（尤指坏事发生）悄无声息地 Research shows that Alzheimer\'s disease develops silently over a decade or longer before causing symptoms. 研究表明，阿尔茨海默病会在十年或更长时间内悄无声息地发展，然后才会出现症状。