How do I purge the Steam Installer?

How do I purge the Steam Installer?

From the related question here the first I did was purging Steam
How do I remove Steam?
The next step would then be to also remove related files in
~/.local/share/Steam and in addition to the answers from above ~/.steam.
One thing makes me believe there still will be more to do. The reason I
purged Steam in the first place was that on other user's account we get a
message to install Steam on every login:

This will definitely not come from files in my home directory, and can
also not be from a system-wide Steam package, as this was purged as can be
seen from the Synaptic window in the background of the shot above. Also in
the other user's accounts there is no ~/.steam or ~/.local/share/Steam
directory. Autostart applications in ~/.config/autostart/ or
/etc/xdg/autostart have no Steam related entries.
Where do I have to look for this "installer" to also remove it? Will there
be any other Steam-related files cluttering my drives?
Here is running 12.04 LTS amd64 on a productive desktop. Steam was
installed initially from the Software Center.