I could not find any occurrences of links to content/browse in any of our default templates except when your already in browse view. (You need to perform a post to get into content/browse by default as far as I can see, and crawlers don't post forms). So, do you have link to content/browse in your content or templates by any chance?
Nope - just checked to be sure. We do not use content/browse anywhere in our templates... Could it be some default ini setting from old times - we have used eZ since 2003...
It may be foolish to ignore the simplest solution in this case. The robots.txt file is the universally accepted way to prevent this concern in general. Ignoring the trivial robots.txt file configuration which would prevent this problem outright is worth completing.
Afterward you could change down other possible causes internally (ghosts/shadows) with free time as needed.
Updated: Useful related links, http://ezpedia.org/ez/robot_txt
Only "solution" we can do in eZ Publish would be to add an additional permission rule for content/browse, so you can control access to it independently of content/read permissions.
If you don't do editing in front end siteaccess, then you can always disable the view in addition to the mentioned robots.txt suggestion.
