finding是现在分词，之后接宾语从句 that Meng Ke was playing truant。逗号前面不是句子，而是现在分词短语作原因状语，the mother was indignant.是主句。又如：

Not knowing the language, she failed to understand what they were talking about. 由于不懂他们的语言，所有她没听懂他们在说些什么。

▲看来你对现在分词作状语不怎么了解，请参考以下链接：

http://www.yygrammar.com/Article/201512/4643.html