404 errors on restricted pages

Webmaster tools flagged quite a few 404 errors on member only pages on http://www.youngwomeninenergy.com

fyi - one of the urls flagged with a 404 error:

Search engines do not have access to these member pages so I'm a little perplexed as to how this is happening.

The site has been inundated with spam lately. Wondering if perhaps it is possible for a removed member to have triggered the 404 while attempting to navigate the site after their account was deleted.

Do you think that is the cause or something else?