You May Be Making This Mistake Now With Copyscape

I had a content of 582 words which was written by outsourcer claiming it to be unique. I scanned it and Copyscape returned no results. Then i took part of content which seemed suspicious duplicate content consisting of 264 words, scanned it and only then Copyscape found URL where original content was located.

Here are 2 files i used:
http://www.filetolink.com/8c65d91e

File 336.txt contains 264 words which are part of whole article and file 588.txt contains 582 words which constitute full article.

Here are my steps:

1. I scanned 582 word content and here are results:
http://i1099.photobucket.com/albums/...2wordsscan.png

2. I scanned 264 word content and Copyscape found 2 URLs:
http://i1099.photobucket.com/albums/...4wordsscan.png

3. First URL in results was found to be containing 233 words matching 88% of that content:
http://i1099.photobucket.com/albums/...atecontent.png

Here is question i asked Copyscape staff:

Here is their reply:

I am disappointed with Copyscape. I always relied on them and was checking full articles. If i understood correctly there are rare cases when scanning full article won't return results even if there is part of plagiarized content. I had to re-scan over 50 articles 2-3 times to make sure nothing else is plagiarized, luckily there were none.

So my advice is to scan content which you suspect to be duplicate, or not to scan more than 250 words of content in a single scan.

UPDATE: Here are online tools for detecting plagiarised content (some are good and some are not, but i sorted them from effective to least effective, however you should test each and find which works best):

http://plagium.com
http://searchenginereports.net/articlecheck.aspx
http://www.articlechecker.com/checker.php
http://www.dustball.com/cs/plagiarism.checker
http://www.10dollararticles.com/adc.htm

DupeFree pro software is currently in its last beta testing stages soon to be released.

I also recommend scanning with multiple tools to be safe.