Sounds like you might be peeking at the Data Summary button on a blank Search screen.
If I remember correctly, that is going to show details on the indexes your ID searches by default. Since you likely need to put index=os into a search to get that data, it won't appear on that table.
You can workaround this though, that table just runs the metadata command, so you could take a step into your Splunk Ninja training by trying your hand at:
| metadata index=* type=sourcetypes
Learn more at Search Reference: metadata
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