Alternate Data Streams (ADS for short) are a seldom used feature of NTFS. In an ADS, additional content can be added to files in the form of streams. Metadata is often stored this way.
Few disk space management tools, however, manage to show those streams. Sadly, the Windows Explorer, too, fails to display the correct file size of a file carrying an Alternate Data Stream. A log file presented with a size of 5 KB could contain a video file of 6 GB without you ever noticing.
SpaceObServer finds Alternate Data Streams and presents them in an extra column.
The determination of Alternate Data Streams results in longer scans. Since ADS are seldom used, this option is deactivated by default. You can activate it in the "Configure Scans" dialog.