Error logs very thin with proxypass

Hi,
Did all the obvious searches and drew a blank on this:
Using apache 2.0.50 as a frontend that's configured with multiple Virt
Hosts, each of which does a proxypass to a variety of backend servers.
Each Virt Host on this frontend proxypass server is configured with its
own access log and error log.

Error responses from the backend servers are logged in the various
frontend server's access logs with the appropriate error codes, but
hardly any of these "bad" backend server responses appear in the
frontend server's error logs.

They DO appear in the error logs of the particular backend server which
suffered the error (as one would expect) but there, they're of limited
use since the IP always = the frontend server.

I'm getting around the problem for now by just searching the access
logs of the frontend server for 404 responses (which are the main ones I
want to email to the various responsible authors) - but I can't help
wondering if this lack of useful data in the error logs of a proxypass
system is normal?

Tony