Actually, I have run this program on my laptop, which is secured with Microsoft Security Essentials & USB Disk Security, but it was undetected.
Then I tested it in my institute's Computers (wishing to find any weakness). My friends use crazy anti-viruses in their PCs but neither of them could catch it. It helped me to make some changes though. I of course have removed them later. :) (It was more than a year ago.)
As I have not tested the program rigorously in all environments, I used the phrase- "almost undetectable".
Thanks a lot for your query.