[Zope] What is "favicon.ico" and why is it an error?

I have a few Zope sites email me any error reports and both of the sites (different machines) get this error about two or three times a day. Traceback: Traceback (innermost last): File /usr/local/zope/lib/python/ZPublisher/Publish.py, line 151, in publish File /usr/local/zope/lib/python/Products/SiteAccess/ChangeBehaviors.py,line 124, in traverse File /usr/local/zope/lib/python/OFS/Application.py, line 249, in __bobo_traverse__ (Object: ApplicationDefaultPermissions) File /usr/local/zope/lib/python/ZPublisher/HTTPResponse.py, line 511, in notFoundError As it looks for this "favicon.ico" Anyone seen this? It is happening on two different instatallations (2.1.4, 2.1.6) on RH Linux 6.1 Thanks, J