Me, and probably a few other Russian people here, are not closely familiar with this phrase, so you might have provided a better explanation of what exactly it means.
For "неожиданно появляться", I came up with these:
Появился из ниоткуда.
Как с потолка\неба свалился.
Как из-под земли вырос.
However, after doing some googling, I found another definition:
"come/crawl out of the woodwork
to appear after being hidden or not active for a long time, especially in order to do something unpleasant.
After you've been in a relationship for a long while, all sorts of little secrets start to come out of the woodwork.
Racists and extreme nationalists are crawling out of the woodwork to protest at the sudden increase in the number of immigrants."
For this I would say something like
... секреты начинают всплывать на поверхность.
Повсюду объявляются рассисты и националисты (лезут изо всех щелей)...