Yes many idioms like 杯弓蛇影 are written language, which are not very often used, and no one use them when speaking. 杯弓蛇影 means someone is unnecessarily suspicious and afraid. If the suspicion is not self-made, if there's real risk, fatal or not, then I think it's reasonable and necessary to get worried.

Snake-like Reflection of a Bow in the Cup Nearly 2,000 years ago, during the Han Dynasty, there was a county magistrate called Ying Chen. One summer day, he invited his secretary Du Xuan to his house and treated him with wine. On the north wall of the room hung a red bow. It was reflected in Du Xuan's cup. Du Xuan took the reflection for a squirming snake. He was very frightened but he dared not turn down Ying Chen's offer because Ying was his superior. He had to swallow the wine with his eyes closed. When he was back at home he felt so painful in his chest and stomach that he could hardly eat and drink any more. He sent for the doctor and took some medicine, but nothing could cure him. When Ying Chen asked Du Xuan how he got so seriously ill, Du told him he drank the wine with a snake in his cup the other day. Ying Chen found something strange about that. He returned home , thought hard, but he could not find an answer. Suddenly the shadow thrown by the bow on the wall caught his eye. "That's it!" he shouted. He immediately sent his man to fetch Du Xuan. He seated him where he sat before and offered him a cup of wine. Du Xuan saw the snake-like shadow again. Before Du was scared out of his wits again, Ying Chen said , pointing at the shadow, "The 'snake' in the cup is nothing but a reflection of the bow on the wall!" Now that Do Xuan knew what it was, he felt much easier. And strange enough, his illness disappeared the next moment. This story was later contracted into the idiom "Bei Gong She Ying'. It is used to describe someone who is very suspicious. It is like saying someone is afraid of his own shadow.

Your interpretation is right. some examples to use this idion: 她敏感得很，常常杯弓蛇影，疑东疑西。 看清楚！那只是块石头！别杯弓蛇影，自己吓自己。 你不要杯弓蛇影，自己糊弄自己。

I think this chenyu has no relationship with lossing jobs or dangerous situations. It is used to describe the personality of somebody is too suspicious and think too much the story is talking about a man he drank tea and saw there are a dark shadow in his cup so he though it was a snak and feel unconfortable all nite. However when he double check with his friend and ask why he put a snake in his cup they suddently find that the shadow is from the wall cuz there hanging a bow on the wall, so he just thought too much. My mother is a Chinese teacher hope it is helpful for you!^^

你好，其实我们平时很少用到这个成语，在现代，几乎没有任何场合用到它。