I've got a similar problem. Class annotated with standard @Webservice, @Webmethod prints to my STDERR which I like to keep error free.
Maybe it is a little late for an answer but ... :-).
I guess your webservice client uses java.util.logging. Please check the JRE/lib/logging.properties if a ConsoleHandler is configured. The ConsoleHandler writes everything out to STDERR which JBoss handles as an [ERROR].
Maybe you can configure an other Handler or supress logging messages lower than SEVERE or take a look at