Actually, I have the same problem. The warning sign appeared when I update the packages once and the sign is still there. I have restarted my computer several times but it won't disappear. Using the terminal to update the packages the following message appears:
GPG error : http://dl.google.com stable Release: The following signatures were invalid: NODATA 1 NODATA 2
Failed to fetch
http://dl.google.com/linux/deb/dists/stable/main/binary-i386/Packages.bz2 Sub-process /bin/bzip2
returned an error code (2)
Could you please tell what what the problem is?
Some index files failed to download,
they have been ignored, or old ones used instead.