What are the main applications that use AI to automatically audit expenses?

Interesting topic. I guess for some companies it makes sense but expenses tend to make up such a small amount of an overall budget. I wouldn't think it is worth the effort to micromanage the process for such a small benefit. 

We have built anomaly detection around expense transactions, to alert outliers. Such models can be build using One Class SVM  

I believe you can detect easily (or more easily...) fraud actions on the expense reports.