Although I'm generally a Winforms guy and ASP.NET is a side line, I'll try my best to give an answer (since no one else has so far).
As I understand the process, IIS compiles the code on the fly and executes the assembly within the IIS process. If unhandled exceptions are thrown by the assembly, IIS logs those to the Event Log (because the code is running in the IIS process any unhandled exceptions ultimately come to IIS). If the code handles it's own exceptions and doesn't re-throw them then IIS will never be aware of them and they won't appear in the Event Log -- that is the fault of the coder, not the fault of IIS.
You're seeing .dll compiled errors -- do you mean errors IIS throws while trying to compile the assembly? That would mean the assembly never runs, hence no exceptions in the Event Log.
Hopefully if I'm way off the mark someone will correct me.