The web scanner doesn't have a cache as such, it scans files in the _avast4_ folder in wherever your Windows Temp is located, these files begin with unp and a string of numbers and .tmp. After avast has scanned the file and it is found to be clear it is removed, should a file be considered infected avast alerts and you get the Abort Connection pop-up once you click on that the unp123456.tmp file is also removed from that folder.
Clear your browsers cache (temporary internet files). However, nothing should have got into the cache if it was detected by the web shield, but if your browser uses your cache to load the site again it may well load without alerting as the infected element won't be in the cache..