In the days since, the Shanghai Composite has barely budged, rising 1.2 per cent this week.
【翻译】自从（上文提到的事情）以来的这些交易日里，上证综指几乎没有变化，本周上升1.2%。
since 用作介词或副词均可，后面跟名词就是介词，省略了名词就是副词。本句就是 since 后面省略了成分后的副词用法，作定语修饰 in the days。再举几个副词用法的例句：
She fell sick last week and has been in bed since.  上星期她病了，一直卧床不起。
He argued with his father and has run afoul of him ever since.  他和他的父亲争辩，从此便得罪了父亲。
I went to the U.S. in 1956 and have lived there ever since.  我1956年去了美国，从那以后就一直住在那。