Just FYI: Monitoring filesystem events is a much more efficient means of accomplishing this goal than doing "full sweeps" on a recurring basis. It is the same mechanism that Dropbox uses to do its magic, for example (as does Goodsync if you set the sync up in that mode).
This allows MC to respond to just changes, rather than doing disk-intensive scans to check each and every possible subfolder for new content. It also improves latency (as you don't have to wait for the recurring "schedule" to arrive, and then wait for the scan to complete). Because of the variety of "depths" of possible folder structures (and complexity within each folder), and sheer sizes of many user's MC Libraries, it wouldn't be practical for MC to schedule a recurring scan. The timing of the scan would have to depend on the complexity of your Library and storage structure. Full disk scans can also be intensive, and you might not want it to happen right while you're watching a movie (especially if you enable the automatic thumbnailing and audio analysis).
Using the events-based approach is better in essentially every conceivable way. Except, that some NAS vendors don't fully implement SMB sharing (or do weird things under the covers where the implementation is broken), and they don't work. But, the same would apply to using these modes with many other similar software applications, so I think that's on those NAS vendors...
If you are using local disks, or those shared by another regular Windows box, and you're seeing this behavior, then something is broken. Did you follow any "services tweak guides" for Windows or anything like that?