Text tracks(.smi files) added to video files should be indexed and searching for keywords from the text tracks should bring up the assets whose text tracks have that keyword in them. Versions of Telescope before 2020.1 had a bug due to which this feature didn't work as described.
All search on Telescope is handled by Solr, and Solr creates and maintains text-based indexes for all the assets in the Telescope database. The indexes are kept up-to-date by sending text updates to Solr. When text tracks are added to an asset, Telescope wasn't sending these updates to Solr, resulting in the assets not appearing for keywords from the text tracks. But these text tracks would get sent to Solr on other text update events such as updating the metadata, after which the assets would show up in search for terms from the text tracks.
Steps To Fix
This issue was fixed with Telescope Executive 2020.1. It is recommended to upgrade to this or a later version. If your TeleScope installation has customizations or you're an OnDemand customer, please contact your Account Manager or support for further assistance with upgrading.
Once updated, add a text track to an asset and search for a word from the text track and the asset will show up.