Answer:
Windows doesn't search the contents of removable drives automatically or the file is encrypted.
Explanation:
"With Windows 7, you can configure the search indexer to not only index any file you want, but also to index the file contents.
By default, the Search Indexer in Windows 7/8/10 indexes the most common locations where your files would be stored, i.e. all libraries, everything in your User folder, and e-mail. If this is not enough, you can add or remove index locations easily."
Reference: Kishore, Aseem. “Windows 7/8/10 File Search Indexing Options.” Help Desk Geek, Help Desk Geek, 14 May 2019