It is quite easy to add something hidden to a page.
Within a form you can use <input type="hidden" ...> but within a page you could use <div class="evil_hiiden_thing">... </div> and then style it to not display
For example, this is Drupal code
<a href="#" class="element-invisible element-focusable">Jump to Navigation</a>
is styled like so
It's there to help screen readers jump to the site navigation.
Copying without referencing the source is plagiarism; I would at least be tempted to visit some of those sites to check my suspicions were correct and then complain to the article writer and any obvious site editor/webmaster. They are after all, benefiting from your bandwidth and if you are planning to make a career out of article writing then it's important you get properly recognised.
Make sure you have the content copyright clearly stated or if you are applying a Creative Commons license clearly state that. Clearly state your policy on the sharing and re-use of article data,
If they don't make the proper reference, you could always blacklist their IP addresses/domain names and thus reduce the hit on your bandwidth? I'd do that at the firewall level.
If you are using an Apache server then please note that access control has changed from mod_access since Apache v2.1, http://httpd.apache.org/docs/2.0/mod/mod_access.html , to http://httpd.apache.org/docs/2.2/howto/access.html .