Comment 3 for bug 983559

The reason this is occurring in precise is that the flashplugin-installer was converted to use the update-notifier package-data-downloader hook which unfortunately does not take the apt proxy configuration settings into account.

In versions before precise, the script that did this formerly in the postinst did the following:

   APT_PROXIES=$(apt-config shell \
                http_proxy Acquire::http::Proxy \
                https_proxy Acquire::https::Proxy \
                ftp_proxy Acquire::ftp::Proxy \
   )

  if [ -n "$APT_PROXIES" ]; then
                eval export $APT_PROXIES
  fi

The package-data-downloader utility should do something similar. Moving this bug over to the update-notifier package.