This page gives you tips on what to avoid and, possibly, how. And the log tells you which user became root and for what command, too. You can still reply as root - but it is not recommended.
Some systems give users a default signature.
And some people do not seem to be aware that they are appending it as their software does not not tell them about it. Many programs are buggy - some even intentional. Some programs even send your disk data away via email.