Thanks for feedback!
Current implementation of the Quick Filter features, show computers that have been already fetched. It doesn't pull entries from an AD to perform filtering.
This would explain why you think it doesn't search in sub-OUs, it because computers for those OUs haven't been fetched.
We are planning to implement "fetch from AD and filter while your are typing" feature in our of the upcoming builds.
What you can do, for now, is to select the All Computers folder and then use the Quick Search feature. But I understand that this is not the perfect solution and there is an extra step involved.
Kindest Regards,SmartCode Solutions Support